refactor: clean up themes, templates, and add comprehensive README

- Themes now only override CSS variables (no selector overrides)
- Removed duplicate styles from header.tmpl (just loads Inter font now)
- Cleaned up logo SVG (simpler, no overlapping text)
- Improved home page with proper hero layout and sign-in/explore buttons
- Added detailed README documenting the setup, deployment, and design principles

public/assets/img/logo.svg

@@ -1,12 +1,10 @@
 <svg xmlns="http://www.w3.org/2000/svg" width="200" height="200" viewBox="0 0 200 200">
   <defs>
-    <linearGradient id="mgitGradient" x1="0%" y1="0%" x2="100%" y2="100%">
-      <stop offset="0%" style="stop-color:#2563eb;stop-opacity:1" />
-      <stop offset="100%" style="stop-color:#1e40af;stop-opacity:1" />
+    <linearGradient id="mgitGrad" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" stop-color="#3b82f6"/>
+      <stop offset="100%" stop-color="#1e40af"/>
     </linearGradient>
   </defs>
-  <rect width="200" height="200" fill="url(#mgitGradient)" rx="24"/>
-  <circle cx="100" cy="100" r="65" fill="#ffffff" opacity="0.95"/>
-  <text x="100" y="125" font-family="Inter, -apple-system, sans-serif" font-size="56" font-weight="700" fill="#2563eb" text-anchor="middle" letter-spacing="-0.02em">m</text>
-  <text x="100" y="155" font-family="Inter, -apple-system, sans-serif" font-size="24" font-weight="600" fill="#1e40af" text-anchor="middle" letter-spacing="-0.01em">Git</text>
+  <rect width="200" height="200" fill="url(#mgitGrad)" rx="40"/>
+  <text x="100" y="138" font-family="Inter, -apple-system, system-ui, sans-serif" font-size="120" font-weight="700" fill="#ffffff" text-anchor="middle" letter-spacing="-6">m</text>
 </svg>