Add PWA support and mobile responsive layout

- PWA manifest, service worker (stale-while-revalidate for static assets,
  network-only for API), and app icons for installability
- Root-scoped /manifest.json and /sw.js routes in FastAPI
- New mobile.css with responsive breakpoints at 768/600/400px:
  fixed bottom tab bar on phones, single-column cards, full-screen modals,
  compact header toolbar, touch-friendly targets
- Fix modal-content-wide min-width overflow on small screens
- Update README with Camera, OpenRGB, and PWA features

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/manifest.json

@@ -0,0 +1,28 @@
+{
+    "name": "LED Grab",
+    "short_name": "LED Grab",
+    "description": "WLED ambient lighting controller based on screen content",
+    "start_url": "/",
+    "display": "standalone",
+    "background_color": "#1a1a1a",
+    "theme_color": "#4CAF50",
+    "orientation": "any",
+    "icons": [
+        {
+            "src": "/static/icons/icon-192.png",
+            "sizes": "192x192",
+            "type": "image/png"
+        },
+        {
+            "src": "/static/icons/icon-512.png",
+            "sizes": "512x512",
+            "type": "image/png"
+        },
+        {
+            "src": "/static/icons/icon-512-maskable.png",
+            "sizes": "512x512",
+            "type": "image/png",
+            "purpose": "maskable"
+        }
+    ]
+}