perf(share): 单题卡二维码同样提到 5px/模块

单题卡右侧卡片只有 210×255，二维码最大只能到 148px（37 模块 = 4px/模块），
缩到 480 宽照样解不出。

- QQRCardW 210 → 236、QQRCardH 255 → 300、QQRSize 150 → 190（37 模块 → 185px）
  卡片仍居中对齐（x 679、y 120），左侧文字区随 QQRCardX 派生，自动收窄 26px
- 二维码卡片绘制抽成 drawQuestionQRCard，可在测试里直接调用出图
- 新增 TestQuestionQRCardLayout：二维码必须正方形、模块 ≥5px、四边留白 ≥20px、
  居中偏差 ≤1px，TEST_PREVIEW=1 输出整卡 PNG；把 QQRSize 改回 150 立刻 FAIL

实测：二维码 185px（5px/模块）、居中偏差 0.5px、上下留白 34px；
gozxing 在 960 / 720 / 480 / 320 宽下都能解出正确链接。

route/question/share.go

@@ -913,11 +913,11 @@ const (
 	QAnswerMaxLine   = 7
 
 	// 右侧二维码卡片
-	QQRCardW = 210
-	QQRCardH = 255
+	QQRCardW = 236
+	QQRCardH = 300
 	QQRCardX = QContentX + QContentW - QQRCardW
 	QQRCardY = QContentY + (QContentH-QQRCardH)/2
-	QQRSize  = 150
+	QQRSize  = 190
 
 	// 内容区最大宽度（左侧，避开二维码卡片）
 	QTextMaxW = QQRCardX - QContentX - 30
@@ -925,6 +925,29 @@ const (
 	QWatermarkY = CardHeight - 30
 )
 
+// drawQuestionQRCard 绘制单题卡右侧的二维码卡片：二维码与提示语整体垂直居中。
+func drawQuestionQRCard(canvas *image.RGBA, user *db.User, question *db.Question, logo image.Image, fontCtx *freetype.Context) error {
+	cardRect := image.Rect(QQRCardX, QQRCardY, QQRCardX+QQRCardW, QQRCardY+QQRCardH)
+	drawRoundedRect(canvas, cardRect, color.NRGBA{245, 245, 245, 255}, 20)
+	drawRoundedRect(canvas, cardRect, ColorWhite, 20)
+
+	questionURL := fmt.Sprintf("%s/_/%s/%d", conf.App.ExternalURL, user.Domain, question.ID)
+	qr, err := generateQR(questionURL, logo, QQRSize)
+	if err != nil {
+		return err
+	}
+
+	const qrToHintGap, hintLineHeight = 30, 17
+	qrSize := qr.Bounds().Dx()
+	contentH := qrSize + qrToHintGap + hintLineHeight
+	qrX := QQRCardX + (QQRCardW-qrSize)/2
+	qrY := QQRCardY + (QQRCardH-contentH)/2
+	draw.Draw(canvas, image.Rect(qrX, qrY, qrX+qrSize, qrY+qrSize), qr, image.Point{}, draw.Over)
+
+	drawTextCentered(canvas, "扫码查看回答", QQRCardX+QQRCardW/2, qrY+qrSize+qrToHintGap+hintLineHeight, FontSizeQRHint, ColorHintGray, fontCtx)
+	return nil
+}
+
 // 单题分享图结果缓存，TTL 30 分钟，上限 16 条（1C1G 内存友好）。
 var questionShareCache = newTTLCache(16)
 
@@ -1066,27 +1089,11 @@ func QuestionShareCard(ctx context.Context) {
 	_ = textY
 
 	// 右侧二维码卡片
-	cardRect := image.Rect(QQRCardX, QQRCardY, QQRCardX+QQRCardW, QQRCardY+QQRCardH)
-	drawRoundedRect(canvas, cardRect, color.NRGBA{245, 245, 245, 255}, 20)
-	drawRoundedRect(canvas, cardRect, ColorWhite, 20)
-
-	questionURL := fmt.Sprintf("%s/_/%s/%d", conf.App.ExternalURL, user.Domain, question.ID)
-	qr, err := generateQR(questionURL, avImg, QQRSize)
-	if err != nil {
+	if err := drawQuestionQRCard(canvas, user, question, avImg, fontCtx); err != nil {
 		http.Error(ctx.ResponseWriter(), "二维码生成失败", http.StatusInternalServerError)
 		logrus.WithContext(ctx.Request().Context()).WithError(err).Error("Failed to generate QR code for question share card")
 		return
 	}
-	qrSize := qr.Bounds().Dx()
-	qrDrawX := QQRCardX + (QQRCardW-qrSize)/2
-	// 二维码与提示语整体垂直居中（已删除底部冗余的长链接，卡片更简洁）
-	const qrToHintGap, hintLineHeight = 30, 17
-	contentH := qrSize + qrToHintGap + hintLineHeight
-	qrDrawY := QQRCardY + (QQRCardH-contentH)/2
-	draw.Draw(canvas, image.Rect(qrDrawX, qrDrawY, qrDrawX+qrSize, qrDrawY+qrSize), qr, image.Point{}, draw.Over)
-
-	hintY := qrDrawY + qrSize + qrToHintGap + hintLineHeight
-	drawTextCentered(canvas, "扫码查看回答", QQRCardX+QQRCardW/2, hintY, 17, ColorHintGray, fontCtx)
 
 	// 底部水印
 	drawTextCentered(canvas, "Powered by 狼的提问箱", CardWidth/2, QWatermarkY, FontSizeFooter, ColorFooterGray, fontCtx)