Add favicon matching the in-app logo icon

src/app/icon.svg — Next.js's file-convention route, auto-served and linked
in <head> (takes precedence over the existing default favicon.ico for
modern browsers). Same accent-colored rounded square + white Gamepad2
glyph as the header logo.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GteWhnWKTmnXcsd5jx6H7u

src/app/icon.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="32" height="32" viewBox="0 0 32 32">
+  <rect width="32" height="32" rx="8" fill="#4f46e5"/>
+  <g transform="translate(7,7) scale(0.75)" fill="none" stroke="white" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round">
+    <line x1="6" x2="10" y1="11" y2="11"/>
+    <line x1="8" x2="8" y1="9" y2="13"/>
+    <line x1="15" x2="15.01" y1="12" y2="12"/>
+    <line x1="18" x2="18.01" y1="10" y2="10"/>
+    <path d="M17.32 5H6.68a4 4 0 0 0-3.978 3.59c-.006.052-.01.101-.017.152C2.604 9.416 2 14.456 2 16a3 3 0 0 0 3 3c1 0 1.5-.5 2-1l1.414-1.414A2 2 0 0 1 9.828 16h4.344a2 2 0 0 1 1.414.586L17 18c.5.5 1 1 2 1a3 3 0 0 0 3-3c0-1.545-.604-6.584-.685-7.258-.007-.05-.011-.1-.017-.151A4 4 0 0 0 17.32 5z"/>
+  </g>
+</svg>