feat: add PWA support (installable app)

- Web app manifest with name, icons, theme color, standalone display
- Service worker with stale-while-revalidate caching strategy
- 192px and 512px PNG icons generated from favicon.svg
- Apple-specific meta tags for iOS home screen support
- Register service worker on page load

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/sw.js

@@ -0,0 +1,33 @@
+const CACHE_NAME = 'reaktor-v1';
+
+self.addEventListener('install', (e) => {
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', (e) => {
+  e.waitUntil(
+    caches.keys().then(keys =>
+      Promise.all(keys.filter(k => k !== CACHE_NAME).map(k => caches.delete(k)))
+    )
+  );
+  self.clients.claim();
+});
+
+self.addEventListener('fetch', (e) => {
+  // Only cache GET requests, skip API calls
+  if (e.request.method !== 'GET') return;
+
+  e.respondWith(
+    caches.match(e.request).then(cached => {
+      const fetching = fetch(e.request).then(response => {
+        if (response.ok) {
+          const clone = response.clone();
+          caches.open(CACHE_NAME).then(cache => cache.put(e.request, clone));
+        }
+        return response;
+      }).catch(() => cached);
+
+      return cached || fetching;
+    })
+  );
+});