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/app.js

@@ -1,4 +1,4 @@
-const VERSION = '0.8.1';
+const VERSION = '0.8.2';
 const HB_LIMIT = 48;
 const UPTIME_DAYS = 90;
 const BROWSER_CONCURRENCY = 6;

public/assets/deco-bicycle.svg

@@ -1,35 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 260 150" fill="none">
-  <title>公路车（简笔画 · 自绘，白子的爱车意象）</title>
-  <g stroke="#2ea8dc" stroke-width="3" stroke-linecap="round" stroke-linejoin="round">
-    <!-- 车轮 -->
-    <circle cx="58" cy="96" r="42"/>
-    <circle cx="58" cy="96" r="4" fill="#2ea8dc" stroke="none"/>
-    <circle cx="202" cy="96" r="42"/>
-    <circle cx="202" cy="96" r="4" fill="#2ea8dc" stroke="none"/>
-  </g>
-  <g stroke="#8fd3f2" stroke-width="1.6" stroke-linecap="round">
-    <!-- 辐条 -->
-    <line x1="58" y1="56" x2="58" y2="136"/><line x1="18" y1="96" x2="98" y2="96"/>
-    <line x1="30" y1="68" x2="86" y2="124"/><line x1="86" y1="68" x2="30" y2="124"/>
-    <line x1="202" y1="56" x2="202" y2="136"/><line x1="162" y1="96" x2="242" y2="96"/>
-    <line x1="174" y1="68" x2="230" y2="124"/><line x1="230" y1="68" x2="174" y2="124"/>
-  </g>
-  <g stroke="#1c2f4d" stroke-width="4" stroke-linecap="round" stroke-linejoin="round">
-    <!-- 车架 -->
-    <path d="M58 96 L110 96 L142 52 L202 96"/>
-    <path d="M110 96 L142 52"/>
-    <path d="M110 96 L146 100"/>
-    <!-- 前叉 + 车把 -->
-    <path d="M202 96 L176 46"/>
-    <path d="M164 40 L190 46"/>
-    <!-- 座管 + 车座 -->
-    <path d="M142 52 L130 34"/>
-    <path d="M118 32 L142 34"/>
-    <!-- 曲柄 + 脚踏 -->
-    <circle cx="146" cy="100" r="10"/>
-    <path d="M146 100 L162 116"/>
-  </g>
-  <g stroke="#2ea8dc" stroke-width="3" stroke-linecap="round">
-    <path d="M96 100 L104 100" opacity="0.9"/>
-  </g>
-</svg>

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 ---------- */