Add site icons for favicon and home screen shortcuts

Include favicon, apple-touch-icon, web manifest, and navbar logo for bookmarks and PWA-style shortcuts.

Co-authored-by: Cursor <cursoragent@cursor.com>

assets/site/favicon.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" role="img" aria-label="S">
+  <defs>
+    <linearGradient id="bg" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" stop-color="#0f3460"/>
+      <stop offset="100%" stop-color="#1a1a2e"/>
+    </linearGradient>
+  </defs>
+  <rect width="512" height="512" rx="112" fill="url(#bg)"/>
+  <circle cx="256" cy="256" r="196" fill="none" stroke="#d4af37" stroke-width="10" opacity="0.85"/>
+  <text x="256" y="292" text-anchor="middle" font-size="220" font-family="'KaiTi','STKaiti','SimSun',serif" fill="#f5f5f5">S</text>
+</svg>

assets/site/site.webmanifest

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+{
+  "name": "DAO DE JING",
+  "short_name": "道德经",
+  "description": "传统文化典籍资料库",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#1a1a2e",
+  "theme_color": "#0f3460",
+  "icons": [
+    {
+      "src": "/apple-touch-icon.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/favicon.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    }
+  ]
+}