Add PWA manifest, app icons, and favicon

D20-themed icon in app color scheme for home screen installation,
favicon, and apple-touch-icon. Standalone display mode makes the
app feel native when launched from home screen.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/public/favicon.svg

@@ -0,0 +1,21 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="32" height="32" viewBox="0 0 32 32">
+  <defs>
+    <linearGradient id="f" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0%" stop-color="#60a5fa"/>
+      <stop offset="100%" stop-color="#2563eb"/>
+    </linearGradient>
+  </defs>
+  <g transform="translate(16 15) scale(1.55)" fill="none" stroke="url(#f)" stroke-width="1.4" stroke-linecap="round" stroke-linejoin="round">
+    <polygon points="8.18 4.76 8.18 -4.74 -0.01 -9.49 -8.19 -4.74 -8.19 4.76 -0.01 9.51 8.18 4.76" fill="url(#f)" fill-opacity="0.15"/>
+    <polygon points="-0.01 -9.49 -4.75 -2.74 4.75 -2.74 -0.01 -9.49" fill="url(#f)" fill-opacity="0.25"/>
+    <polygon points="-0.01 5.44 8.18 4.76 4.75 -2.74 -0.01 5.44" fill="url(#f)" fill-opacity="0.2"/>
+    <polygon points="-0.01 5.44 -4.75 -2.74 -8.19 4.76 -0.01 5.44" fill="url(#f)" fill-opacity="0.1"/>
+    <line x1="-4.75" y1="-2.74" x2="-8.19" y2="-4.74"/>
+    <line x1="8.18" y1="-4.74" x2="4.75" y2="-2.74"/>
+    <line x1="-0.01" y1="5.44" x2="-0.01" y2="9.51"/>
+    <polygon points="8.18 4.76 8.18 -4.74 -0.01 -9.49 -8.19 -4.74 -8.19 4.76 -0.01 9.51 8.18 4.76"/>
+    <polygon points="-0.01 -9.49 -4.75 -2.74 4.75 -2.74 -0.01 -9.49"/>
+    <polygon points="-0.01 5.44 8.18 4.76 4.75 -2.74 -0.01 5.44"/>
+    <polygon points="-0.01 5.44 -4.75 -2.74 -8.19 4.76 -0.01 5.44"/>
+  </g>
+</svg>