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/manifest.json

@@ -0,0 +1,15 @@
+{
+  "name": "Reaktor — MontLab Modular Synth",
+  "short_name": "Reaktor",
+  "description": "Modular synthesizer & SynthQuest puzzle game",
+  "start_url": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#08080f",
+  "theme_color": "#00e5ff",
+  "icons": [
+    { "src": "/favicon.svg", "sizes": "any", "type": "image/svg+xml", "purpose": "any" },
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png" }
+  ]
+}