fix: use the ChefHat logo mark as favicon

app/icon.svg (Next.js auto-detected favicon convention) now uses the
same ChefHat glyph as the nav header logo instead of the default
Next.js placeholder. Also updated the PWA manifest icons to match so
favicon/logo/install-icon are all the same mark.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/app/icon.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="32" height="32">
+  <rect width="24" height="24" rx="5" fill="#18181b"/>
+  <g transform="translate(2.5, 2.5) scale(0.79)" fill="none" stroke="#fafafa" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+    <path d="M17 21a1 1 0 0 0 1-1v-5.35c0-.457.316-.844.727-1.041a4 4 0 0 0-2.134-7.589 5 5 0 0 0-9.186 0 4 4 0 0 0-2.134 7.588c.411.198.727.585.727 1.041V20a1 1 0 0 0 1 1Z"/>
+    <path d="M6 17h12"/>
+  </g>
+</svg>