Make the frontend an installable PWA

Add a web app manifest and an install-only service worker (no offline
caching: live state always comes from the server and hashed asset names
change every build), registered from the WASM bundle. The worker must
live at the site root for its scope to cover the app, and Granian only
serves /static and /assets, so tavolo/static.py answers
/manifest.webmanifest and /sw.js from STATIC_DIR with the content types
browsers require; index.html gains the manifest link, icons, theme-color
and Apple standalone meta tags.

Icons are committed under web/assets/icons/ with vector sources and a
make-icons.sh regeneration script, mirroring the card art workflow.

web/assets/icons/icon.svg

@@ -0,0 +1,15 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="512" height="512" viewBox="0 0 512 512">
+  <!-- Tavolo app icon: the gold spade from the navbar on dark felt.
+       The spade glyph is Bootstrap Icons' "suit-spade-fill" (MIT), the
+       same license as the vendored Bootstrap CSS. Rasterized into the
+       PNG sizes by make-icons.sh (rsvg-convert); not used directly at
+       runtime except as the SVG favicon. -->
+  <defs>
+    <radialGradient id="felt" cx="50%" cy="35%" r="80%">
+      <stop offset="0" stop-color="#1d5c38"/>
+      <stop offset="1" stop-color="#14341f"/>
+    </radialGradient>
+  </defs>
+  <rect width="512" height="512" rx="112" fill="url(#felt)"/>
+  <path fill="#e8c547" transform="translate(96 104) scale(20)" d="M7.184 11.246A3.5 3.5 0 0 1 1 9c0-1.602 1.14-2.633 2.66-4.008C4.986 3.792 6.184 2.602 8 0c1.818 2.602 3.016 3.793 4.34 4.992C13.86 6.367 15 7.398 15 9a3.5 3.5 0 0 1-6.184 2.246 20 20 0 0 0 1.06 3.09c.42.897-.155 1.664-1.876 1.664s-2.296-.767-1.876-1.664a20 20 0 0 0 1.06-3.09Z"/>
+</svg>