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/sw.js

@@ -0,0 +1,90 @@
+/**
+ * Service Worker for LED Grab PWA.
+ *
+ * Strategy:
+ *   - Static assets (/static/): stale-while-revalidate
+ *   - API / config requests: network-only (device control must be live)
+ *   - Navigation: network-first with offline fallback
+ */
+
+const CACHE_NAME = 'ledgrab-v1';
+
+const PRECACHE_URLS = [
+    '/',
+    '/static/css/base.css',
+    '/static/css/layout.css',
+    '/static/css/components.css',
+    '/static/css/cards.css',
+    '/static/css/modal.css',
+    '/static/css/calibration.css',
+    '/static/css/dashboard.css',
+    '/static/css/streams.css',
+    '/static/css/patterns.css',
+    '/static/css/automations.css',
+    '/static/css/tutorials.css',
+    '/static/css/mobile.css',
+    '/static/icons/icon-192.png',
+    '/static/icons/icon-512.png',
+];
+
+// Install: pre-cache core shell
+self.addEventListener('install', (event) => {
+    event.waitUntil(
+        caches.open(CACHE_NAME)
+            .then((cache) => cache.addAll(PRECACHE_URLS))
+            .then(() => self.skipWaiting())
+    );
+});
+
+// Activate: clean old caches
+self.addEventListener('activate', (event) => {
+    event.waitUntil(
+        caches.keys()
+            .then((keys) => Promise.all(
+                keys.filter((k) => k !== CACHE_NAME).map((k) => caches.delete(k))
+            ))
+            .then(() => self.clients.claim())
+    );
+});
+
+// Fetch handler
+self.addEventListener('fetch', (event) => {
+    const url = new URL(event.request.url);
+
+    // API and config: always network (device control must be live)
+    if (url.pathname.startsWith('/api/') || url.pathname.startsWith('/config/')) {
+        return; // fall through to default network fetch
+    }
+
+    // Static assets: stale-while-revalidate
+    if (url.pathname.startsWith('/static/')) {
+        event.respondWith(
+            caches.open(CACHE_NAME).then((cache) =>
+                cache.match(event.request).then((cached) => {
+                    const fetchPromise = fetch(event.request).then((response) => {
+                        if (response.ok) {
+                            cache.put(event.request, response.clone());
+                        }
+                        return response;
+                    }).catch(() => cached);
+
+                    return cached || fetchPromise;
+                })
+            )
+        );
+        return;
+    }
+
+    // Navigation: network-first
+    if (event.request.mode === 'navigate') {
+        event.respondWith(
+            fetch(event.request).catch(() =>
+                caches.match('/') || new Response('Offline', {
+                    status: 503,
+                    headers: { 'Content-Type': 'text/plain' },
+                })
+            )
+        );
+        return;
+    }
+});