feat(theme): 左下角公路车也换成简约扁平风插画

- 用图像生成（同一套扁平风格）+ Pillow 抠底，替换自绘线稿 deco-bicycle.svg
- 新素材 public/assets/shiroko-bike.webp（341x200，27KB，透明底）
- 页脚下方装饰带（公路车 / 光环水印 / 人物立绘）视觉语言统一
- README 同步更新元素表、素材来源表与目录树

验证：四套脚本 81 项全通过；1440 宽下 0 横向溢出

public/assets/theme-shiroko.css

@@ -79,18 +79,18 @@ body[data-theme="shiroko"]::after {
   background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 240 130'><g stroke='%232ea8dc' stroke-opacity='0.11' fill='none' stroke-linecap='round'><ellipse cx='120' cy='64' rx='86' ry='38' stroke-width='11'/><ellipse cx='120' cy='70' rx='48' ry='21' stroke-width='6'/><g stroke-width='7'><line x1='22' y1='64' x2='6' y2='64'/><line x1='218' y1='64' x2='234' y2='64'/><line x1='120' y1='18' x2='120' y2='2'/><line x1='120' y1='110' x2='120' y2='126'/></g></g><g fill='%232ea8dc' fill-opacity='0.11'><circle cx='6' cy='64' r='4.6'/><circle cx='234' cy='64' r='4.6'/><circle cx='120' cy='2' r='4.6'/><circle cx='120' cy='126' r='4.6'/></g></svg>");
 }
 
-/* 左下角：公路车（自绘） */
+/* 左下角：简约风公路车插画（与人物同一套扁平风格） */
 body[data-theme="shiroko"]::before {
   content: "";
   position: fixed;
-  left: 14px;
-  bottom: 10px;
-  width: 156px;
-  height: 90px;
+  left: 8px;
+  bottom: 2px;
+  width: 158px;
+  height: 93px;
   pointer-events: none;
   z-index: 0;
-  opacity: 0.5;
-  background: url("deco-bicycle.svg") no-repeat center / contain;
+  opacity: 0.85;
+  background: url("shiroko-bike.webp") no-repeat center / contain;
 }
 
 /* ---------- 顶栏：天蓝到白 + 底部围巾飘带 ---------- */