feat: add blue airplane favicon using Lucide Plane icon

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/public/favicon.svg

@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#3b82f6"/>
+      <stop offset="100%" stop-color="#1d4ed8"/>
+    </linearGradient>
+  </defs>
+  <rect width="32" height="32" rx="6" fill="url(#bg)"/>
+  <g transform="translate(4,4)" fill="none" stroke="white" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round">
+    <path d="M17.8 19.2 16 11l3.5-3.5C21 6 21.5 4 21 3c-1-.5-3 0-4.5 1.5L13 8 4.8 6.2c-.5-.1-.9.1-1.1.5l-.3.5c-.2.5-.1 1 .3 1.3L9 12l-2 3H4l-1 1 3 2 2 3 1-1v-3l3-2 3.5 5.3c.3.4.8.5 1.3.3l.5-.2c.4-.3.6-.7.5-1.2z"/>
+  </g>
+</svg>