fix: add favicon (SVG + PNG + ICO), fix static MIME types (#613)

Squash-merges PR #613. Adds favicon to the app (was missing entirely — blank tab icon). 1371 tests passing, QA harness green. Review by independent agent (see PR comments). Follow-up commit addresses all three reviewer notes: hoisted _STATIC_MIME to module scope, fixed charset=utf-8 being appended to binary MIME types, confirmed correct MIME types on all three favicon formats.

Co-authored-by: tiansiyuan <tiansiyuan@users.noreply.github.com>

static/favicon.svg

@@ -0,0 +1,20 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
+  <rect width="64" height="64" rx="12" fill="#1a1a1a"/>
+  <defs>
+    <linearGradient id="g" x1="0%" y1="0%" x2="0%" y2="100%">
+      <stop offset="0%" style="stop-color:#F5C542;stop-opacity:1"/>
+      <stop offset="100%" style="stop-color:#D4961C;stop-opacity:1"/>
+    </linearGradient>
+  </defs>
+  <rect x="30" y="10" width="4" height="46" rx="2" fill="url(#g)"/>
+  <path d="M30 18 C24 14, 14 14, 10 18 C14 16, 22 16, 28 20" fill="#F5C542" opacity="0.9"/>
+  <path d="M30 22 C26 19, 18 19, 14 22 C18 20, 24 20, 28 24" fill="#D4961C" opacity="0.8"/>
+  <path d="M34 18 C40 14, 50 14, 54 18 C50 16, 42 16, 36 20" fill="#F5C542" opacity="0.9"/>
+  <path d="M34 22 C38 19, 46 19, 50 22 C46 20, 40 20, 36 24" fill="#D4961C" opacity="0.8"/>
+  <path d="M32 48 C22 44, 20 38, 26 34 C20 36, 18 42, 24 46 C18 40, 22 30, 30 28 C24 32, 22 38, 28 42"
+        fill="none" stroke="#F5C542" stroke-width="2.5" stroke-linecap="round"/>
+  <path d="M32 48 C42 44, 44 38, 38 34 C44 36, 46 42, 40 46 C46 40, 42 30, 34 28 C40 32, 42 38, 36 42"
+        fill="none" stroke="#D4961C" stroke-width="2.5" stroke-linecap="round"/>
+  <circle cx="32" cy="10" r="4" fill="#F5C542"/>
+  <circle cx="32" cy="10" r="2" fill="#FFF8E1" opacity="0.7"/>
+</svg>