feat: update favicon to match dashboard Activity icon

Created SVG icon that matches the Activity/pulse icon used in the app:
- Blue gradient background matching the app theme
- White pulse/activity line icon
- Updated layout.tsx to use the new SVG icon

public/icon.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32" fill="none">
+  <defs>
+    <linearGradient id="gradient" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#2563eb"/>
+      <stop offset="100%" style="stop-color:#1d4ed8"/>
+    </linearGradient>
+  </defs>
+  <rect width="32" height="32" rx="6" fill="url(#gradient)"/>
+  <path d="M8 16h3l2-6 4 12 3-6h2" stroke="white" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+</svg>