Add comprehensive favicon and PWA support

- Created SVG favicons in multiple sizes (16x16, 32x32, 192x192, 512x512)
- Added web app manifest for PWA functionality
- Implemented Apple Touch Icon support for iOS devices
- Added theme color and mobile web app meta tags
- Used gear/cogs design matching UnitForge branding with blue gradient
- Supports modern browsers with SVG and fallback compatibility
- Enables 'Add to Home Screen' functionality on mobile devices

All favicon files use the same blue gradient (#0d6efd to #0b5ed7)
as the main UI theme for consistent branding.

frontend/static/img/favicon-16x16.svg

@@ -0,0 +1,20 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16">
+  <defs>
+    <linearGradient id="gear-gradient-16" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#0d6efd;stop-opacity:1" />
+      <stop offset="100%" style="stop-color:#0b5ed7;stop-opacity:1" />
+    </linearGradient>
+  </defs>
+
+  <!-- Background circle -->
+  <circle cx="8" cy="8" r="7.5" fill="url(#gear-gradient-16)" stroke="#ffffff" stroke-width="0.5"/>
+
+  <!-- Main gear -->
+  <g fill="#ffffff" transform="translate(8,8)">
+    <!-- Simplified gear teeth for 16x16 -->
+    <path d="M-4,-5 L-2,-5 L-2,-6 L-1,-6 L-1,-5 L1,-5 L1,-6 L2,-6 L2,-5 L4,-5 L5,-4 L5,-2 L6,-2 L6,-1 L5,-1 L5,1 L6,1 L6,2 L5,2 L5,4 L4,5 L2,5 L2,6 L1,6 L1,5 L-1,5 L-1,6 L-2,6 L-2,5 L-4,5 L-5,4 L-5,2 L-6,2 L-6,1 L-5,1 L-5,-1 L-6,-1 L-6,-2 L-5,-2 L-5,-4 Z"/>
+
+    <!-- Inner circle -->
+    <circle cx="0" cy="0" r="2" fill="url(#gear-gradient-16)" stroke="#ffffff" stroke-width="0.3"/>
+  </g>
+</svg>