fix(share): 分享卡链接按真实字宽居中，长链接按 host/path 收敛

原实现用字符宽度系数估算文本宽度，把标点/符号一律按 1em 计，
而 URL 里的 : / . _ - 实际只有 0.25~0.35em，整句高估 55%：
15px 的 box.shiroko.one/_/tamakyi 估算 352.5px、真实 227.9px，
越过卡片可用宽 230px 后触发等比缩字号，最终只画到 9.6px 且左偏 40px；
链接越长字号越小（长域名实测 5.3px），整段挤在卡片左侧。

- measureTextWidth 改为逐字累加 HMetric.AdvanceWidth + Kern，
  scale = fontSize*64 与 freetype 在 72 DPI 下的换算一致，测量结果与
  DrawString 返回笔位逐位相等；字体不可用时回退估算
- drawTextCentered 随之修正，底部水印原本偏 9.8px 一并归位
- 新增 displayURL（去协议前缀）/ layoutLink / ellipsizeMiddle：
  单行 15→13px；放不下按 host/path 拆两行 13→11px；host 超宽才中间省略；
  字号下限 11px，行长恒定不超过 QRCardW-40
- 卡片内容高度按链接行数计算，两行时整体重新居中

守门测试见 route/question/share_layout_test.go（测量=笔位、限宽、按着墨
边界判定居中 ±3px、TEST_PREVIEW=1 出图）。

go.mod

@@ -48,6 +48,7 @@ require (
 	go.opentelemetry.io/otel/sdk v1.9.0
 	go.opentelemetry.io/otel/trace v1.10.0
 	golang.org/x/crypto v0.54.0
+	golang.org/x/image v0.13.0
 	golang.org/x/net v0.56.0
 	gopkg.in/gomail.v2 v2.0.0-20160411212932-81ebce5c23df
 	gopkg.in/ini.v1 v1.67.0
@@ -151,7 +152,6 @@ require (
 	go.opentelemetry.io/proto/otlp v0.19.0 // indirect
 	go.uber.org/multierr v1.11.0 // indirect
 	golang.org/x/exp v0.0.0-20260112195511-716be5621a96 // indirect
-	golang.org/x/image v0.13.0 // indirect
 	golang.org/x/mod v0.37.0 // indirect
 	golang.org/x/sync v0.22.0 // indirect
 	golang.org/x/sys v0.47.0 // indirect

route/question/share.go

@@ -24,6 +24,7 @@ import (
 	"github.com/golang/freetype/truetype"
 	"github.com/sirupsen/logrus"
 	qrcode "github.com/skip2/go-qrcode"
+	"golang.org/x/image/math/fixed"
 
 	"github.com/tamakyi/TamaBox/internal/conf"
 	"github.com/tamakyi/TamaBox/internal/context"
@@ -99,6 +100,15 @@ const (
 	FontSizeFooter = 14
 )
 
+// 链接区排版：可用宽度按卡片内边距 20px 计算；字号下限保证长域名仍然可读，
+// 单行放不下时改为 host/path 两行，而不是把字号一路缩到看不清。
+const (
+	LinkMaxWidth      = QRCardW - 40
+	LinkSingleMinSize = 13.0
+	LinkMinSize       = 11.0
+	LinkSizeStep      = 0.5
+)
+
 // 颜色系统
 var (
 	ColorWhite      = color.RGBA{255, 255, 255, 255}
@@ -468,7 +478,8 @@ func ShareCard(ctx context.Context) {
 		sepToLinkGap   = 21
 		linkLineHeight = 15
 	)
-	totalContentH := QRCodeSize + qrToHintGap + hintLineHeight + hintToSepGap + sepHeight + sepToLinkGap + linkLineHeight
+	linkLines, linkFontSize := layoutLink(displayURL(boxURL))
+	totalContentH := QRCodeSize + qrToHintGap + hintLineHeight + hintToSepGap + sepHeight + sepToLinkGap + linkLineHeight*len(linkLines)
 	startY := QRCardY + (QRCardH-totalContentH)/2
 
 	qrDrawX := QRCardX + QRCodeX
@@ -482,14 +493,9 @@ func ShareCard(ctx context.Context) {
 	drawLine(canvas, cardCenterX-90, sepY, cardCenterX+90, sepY, color.NRGBA{200, 200, 200, 180})
 
 	linkY := sepY + sepToLinkGap + linkLineHeight
-	linkText := boxURL
+	for i, line := range linkLines {
-	linkFontSize := float64(FontSizeLink)
+		drawTextCentered(canvas, line, cardCenterX, linkY+i*linkLineHeight, linkFontSize, ColorDarkText, fontCtx)
-	availableW := QRCardW - 45
-	actualW := measureTextWidth(linkText, linkFontSize)
-	if actualW > float64(availableW) {
-		linkFontSize = linkFontSize * float64(availableW) / actualW
 	}
-	drawTextCentered(canvas, linkText, cardCenterX, linkY, linkFontSize, ColorDarkText, fontCtx)
 
 	// 7. 底部水印
 	drawTextCentered(canvas, "Powered by 狼的提问箱", CardWidth/2, FooterY, FontSizeFooter, ColorFooterGray, fontCtx)
@@ -707,12 +713,32 @@ func charPixelWidth(r rune, fontSize float64) float64 {
 	return fontSize * 0.62
 }
 
-// measureTextWidth 用逐字符宽度估算（与 splitText 的 charPixelWidth 一致），
+// measureTextWidth 返回文本在给定字号下的真实推进宽度（像素）。
-// 避免每次测量都创建 freetype.Context 并执行 DrawString（显著降低生成耗时）。
+// 逐字累加字形 advance 与 kern，scale 与 freetype.Context 在 72 DPI 下的换算一致
+// （fontSize * 64）；用它做居中，误差从原来估算的几十像素降到亚像素。
+// 字体不可用时回退到字符宽度估算，保证分享卡不会因字体缺失而完全不可用。
 func measureTextWidth(text string, fontSize float64) float64 {
-	return measureTextEstimate(text, fontSize)
+	ft, err := getFont()
+	if err != nil || ft == nil {
+		return measureTextEstimate(text, fontSize)
+	}
+	scale := fixed.Int26_6(fontSize * 64.0)
+	total := fixed.Int26_6(0)
+	var prev truetype.Index
+	hasPrev := false
+	for _, r := range text {
+		idx := ft.Index(r)
+		if hasPrev {
+			total += ft.Kern(scale, prev, idx)
+		}
+		total += ft.HMetric(scale, idx).AdvanceWidth
+		prev, hasPrev = idx, true
+	}
+	return float64(total) / 64.0
 }
 
+// measureTextEstimate 按字符宽度系数粗估，仅作字体不可用时的回退。
+// 注意：标点/符号按 1em 计，对 URL 会高估 50% 以上，不能用于居中定位。
 func measureTextEstimate(text string, fontSize float64) float64 {
 	total := 0.0
 	for _, r := range text {
@@ -721,6 +747,56 @@ func measureTextEstimate(text string, fontSize float64) float64 {
 	return total
 }
 
+// displayURL 去掉协议前缀与结尾斜杠：二维码里仍编码完整地址，
+// 人眼需要辨认的只是 host 与路径。
+func displayURL(rawURL string) string {
+	s := strings.TrimPrefix(rawURL, "https://")
+	s = strings.TrimPrefix(s, "http://")
+	return strings.TrimSuffix(s, "/")
+}
+
+// layoutLink 把展示用地址收进卡片宽度，返回逐行文本与统一字号。
+// 顺序：单行 15→13px；仍放不下按 host/path 拆两行（13→11px，而不是在
+// host 中间的连字符处断开）；host 自身超宽才做中间省略。
+func layoutLink(text string) ([]string, float64) {
+	maxW := float64(LinkMaxWidth)
+	for size := float64(FontSizeLink); size >= LinkSingleMinSize; size -= LinkSizeStep {
+		if measureTextWidth(text, size) <= maxW {
+			return []string{text}, size
+		}
+	}
+	if i := strings.Index(text, "/"); i > 0 {
+		host, path := text[:i], text[i:]
+		for size := LinkSingleMinSize; size >= LinkMinSize; size -= LinkSizeStep {
+			if measureTextWidth(host, size) <= maxW && measureTextWidth(path, size) <= maxW {
+				return []string{host, path}, size
+			}
+		}
+		return []string{
+			ellipsizeMiddle(host, maxW, LinkMinSize),
+			ellipsizeMiddle(path, maxW, LinkMinSize),
+		}, LinkMinSize
+	}
+	return []string{ellipsizeMiddle(text, maxW, LinkMinSize)}, LinkMinSize
+}
+
+// ellipsizeMiddle 中间省略：保住开头与结尾（域名结尾通常是 TLD，最需要留下）。
+func ellipsizeMiddle(text string, maxWidth, fontSize float64) string {
+	if measureTextWidth(text, fontSize) <= maxWidth {
+		return text
+	}
+	r := []rune(text)
+	for keep := len(r) - 1; keep >= 3; keep-- {
+		head := keep * 6 / 10
+		tail := keep - head
+		s := string(r[:head]) + "…" + string(r[len(r)-tail:])
+		if measureTextWidth(s, fontSize) <= maxWidth {
+			return s
+		}
+	}
+	return text
+}
+
 func drawTextCentered(dst *image.RGBA, text string, cx, y int, fontSize float64, col color.Color, ftCtx *freetype.Context) {
 	ftCtx.SetClip(dst.Bounds())
 	ftCtx.SetSrc(image.NewUniform(col))

route/question/share_layout_test.go

@@ -0,0 +1,210 @@
+package question
+
+import (
+	"image"
+	"image/color"
+	"image/draw"
+	"image/png"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/disintegration/imaging"
+	"github.com/golang/freetype"
+	"github.com/golang/freetype/truetype"
+
+	"github.com/tamakyi/TamaBox/internal/conf"
+)
+
+// 分享卡测试需要一份真实字体；中文站点部署时 conf.App.FontPath 必然存在，
+// 本机开发/CI 缺失时跳过，避免把「没有字体」当成排版错误。
+func TestMain(m *testing.M) {
+	if _, err := os.Stat(conf.App.FontPath); err != nil {
+		for _, candidate := range []string{
+			"C:/Windows/Fonts/NotoSansSC-VF.ttf",
+			"C:/Windows/Fonts/msyh.ttc",
+			"/usr/share/fonts/opentype/noto/NotoSansCJK-Regular.ttc",
+		} {
+			if _, err := os.Stat(candidate); err == nil {
+				conf.App.FontPath = candidate
+				break
+			}
+		}
+	}
+	os.Exit(m.Run())
+}
+
+func requireFont(t *testing.T) *truetype.Font {
+	t.Helper()
+	ft, err := getFont()
+	if err != nil {
+		t.Skipf("no usable font (%v), skip share card layout tests", err)
+	}
+	return ft
+}
+
+// measureTextWidth 必须与 freetype 实际笔位推进一致，否则居中会重新出现偏移。
+func TestMeasureTextWidthMatchesAdvance(t *testing.T) {
+	requireFont(t)
+	cases := []struct {
+		text string
+		size float64
+	}{
+		{"https://box.shiroko.one/_/tamakyi", FontSizeLink},
+		{"box.shiroko.one/_/tamakyi", 13},
+		{"ask.some-quite-long-domain-name.example.com", LinkMinSize},
+		{"Powered by 狼的提问箱", FontSizeFooter},
+		{"扫码向我匿名提问", FontSizeQRHint},
+	}
+	for _, c := range cases {
+		dst := image.NewRGBA(image.Rect(0, 0, 1400, 80))
+		fctx := freetype.NewContext()
+		fctx.SetFont(requireFont(t))
+		fctx.SetDPI(72)
+		fctx.SetFontSize(c.size)
+		fctx.SetSrc(image.NewUniform(color.Black))
+		fctx.SetDst(dst)
+		pen, err := fctx.DrawString(c.text, freetype.Pt(0, 60))
+		if err != nil {
+			t.Fatalf("draw %q: %v", c.text, err)
+		}
+		want := float64(pen.X) / 64.0
+		if got := measureTextWidth(c.text, c.size); got != want {
+			t.Errorf("%q @%.1f: measure=%.2f, advance=%.2f", c.text, c.size, got, want)
+		}
+	}
+}
+
+// 任意长度的链接都必须收进卡片，且不得缩到不可读。
+func TestLayoutLinkFitsCard(t *testing.T) {
+	requireFont(t)
+	urls := []string{
+		"box.shiroko.one/_/tamakyi",
+		"box-dev-api.337845818.best/_/tamakyi",
+		"ask.some-quite-long-domain-name.example.com/_/tamakyi",
+		"questions.tamabox-community.example.org/anon/box/_/a-rather-long-handle-name",
+		"averyveryveryverylongdomainnamethatkeepsgoingandonandonforever.example/_/x",
+	}
+	for _, u := range urls {
+		lines, size := layoutLink(u)
+		if size < LinkMinSize {
+			t.Errorf("%q: font size %.2f below floor %.2f", u, size, LinkMinSize)
+		}
+		if len(lines) > 2 {
+			t.Errorf("%q: got %d lines, want <=2", u, len(lines))
+		}
+		for _, line := range lines {
+			if w := measureTextWidth(line, size); w > float64(LinkMaxWidth) {
+				t.Errorf("%q: line %q is %.1fpx, card allows %dpx", u, line, w, LinkMaxWidth)
+			}
+		}
+	}
+}
+
+// 居中必须落在二维码卡片中线上（按真实着墨边界判断，允许左右侧边距带来的 3px 内偏差）。
+func TestDrawTextCenteredIsCentered(t *testing.T) {
+	requireFont(t)
+	cx := QRCardX + QRCardW/2
+	for _, text := range []string{
+		"box.shiroko.one/_/tamakyi",
+		"box-dev-api.337845818.best/_/tamakyi",
+		"Powered by 狼的提问箱",
+	} {
+		canvas := image.NewRGBA(image.Rect(0, 0, CardWidth, CardHeight))
+		draw.Draw(canvas, canvas.Bounds(), &image.Uniform{ColorWhite}, image.Point{}, draw.Src)
+		fctx := freetype.NewContext()
+		fctx.SetFont(requireFont(t))
+		fctx.SetDPI(72)
+		drawTextCentered(canvas, text, cx, 300, FontSizeLink, ColorDarkText, fctx)
+
+		minX, maxX := -1, -1
+		for x := 0; x < CardWidth; x++ {
+			for y := 250; y < 320; y++ {
+				if r, _, _, a := canvas.At(x, y).RGBA(); a > 40 && r < 0x8000 {
+					if minX < 0 {
+						minX = x
+					}
+					maxX = x
+					break
+				}
+			}
+		}
+		if minX < 0 {
+			t.Fatalf("%q: nothing drawn", text)
+		}
+		if center := (minX + maxX) / 2; abs(center-cx) > 3 {
+			t.Errorf("%q: ink center %d, want %d (±3)", text, center, cx)
+		}
+		if maxX > QRCardX+QRCardW || minX < QRCardX {
+			t.Errorf("%q: ink [%d,%d] escapes card [%d,%d]", text, minX, maxX, QRCardX, QRCardX+QRCardW)
+		}
+	}
+}
+
+func abs(v int) int {
+	if v < 0 {
+		return -v
+	}
+	return v
+}
+
+// 视觉自检：TEST_PREVIEW=1 时把链接区渲成 PNG，人工确认断行与对齐。
+func TestShareCardLinkPreview(t *testing.T) {
+	if os.Getenv("TEST_PREVIEW") == "" {
+		t.Skip("set TEST_PREVIEW=1 to write preview images")
+	}
+	requireFont(t)
+	outDir := os.Getenv("TEST_PREVIEW_DIR")
+	if outDir == "" {
+		outDir = filepath.Join("..", "..", "outputs", "sharecard-preview")
+	}
+	if err := os.MkdirAll(outDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+
+	cases := map[string]string{
+		"short":    "box.shiroko.one/_/tamakyi",
+		"dev":      "box-dev-api.337845818.best/_/tamakyi",
+		"long":     "ask.some-quite-long-domain-name.example.com/_/tamakyi",
+		"verylong": "questions.tamabox-community.example.org/anon/box/_/a-rather-long-handle-name",
+	}
+	for name, display := range cases {
+		canvas := image.NewRGBA(image.Rect(0, 0, QRCardW, QRCardH))
+		draw.Draw(canvas, canvas.Bounds(), &image.Uniform{color.NRGBA{246, 246, 246, 255}}, image.Point{}, draw.Src)
+		drawRoundedRect(canvas, canvas.Bounds(), ColorWhite, 18)
+
+		fctx := freetype.NewContext()
+		fctx.SetFont(requireFont(t))
+		fctx.SetDPI(72)
+
+		lines, size := layoutLink(display)
+		const qrToHintGap, hintLineHeight, hintToSepGap, sepHeight, sepToLinkGap, linkLineHeight = 26, 17, 21, 1, 21, 15
+		total := QRCodeSize + qrToHintGap + hintLineHeight + hintToSepGap + sepHeight + sepToLinkGap + linkLineHeight*len(lines)
+		startY := (QRCardH - total) / 2
+
+		qr, err := generateQR("https://"+display, nil, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		qrResized := imaging.Resize(qr, QRCodeSize, QRCodeSize, imaging.NearestNeighbor)
+		draw.Draw(canvas, image.Rect(QRCodeX, startY, QRCodeX+QRCodeSize, startY+QRCodeSize), qrResized, image.Point{}, draw.Over)
+
+		hintY := startY + QRCodeSize + qrToHintGap + hintLineHeight
+		drawTextCentered(canvas, "扫码向我匿名提问", QRCardW/2, hintY, FontSizeQRHint, ColorHintGray, fctx)
+		sepY := hintY + hintToSepGap
+		drawLine(canvas, QRCardW/2-90, sepY, QRCardW/2+90, sepY, color.NRGBA{200, 200, 200, 180})
+		linkY := sepY + sepToLinkGap + linkLineHeight
+		for i, line := range lines {
+			drawTextCentered(canvas, line, QRCardW/2, linkY+i*linkLineHeight, size, ColorDarkText, fctx)
+		}
+
+		f, err := os.Create(filepath.Join(outDir, name+".png"))
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := png.Encode(f, canvas); err != nil {
+			t.Fatal(err)
+		}
+		f.Close()
+	}
+}