feat(pwa): add Progressive Web App support with iOS compatibility

- Install vite-plugin-pwa and workbox-window for PWA functionality
- Configure Vite with full PWA manifest (name, icons, theme, display)
- Add service worker caching for static assets only (no API cache)
- Create app icons (192x192, 512x512, apple-touch-icon)
- Generate iOS splash screens for multiple device sizes
- Add iOS-specific meta tags (apple-mobile-web-app-capable, etc.)
- Implement InstallPWA component with dual platform support:
  - Android/Chrome: beforeinstallprompt event with custom UI
  - iOS Safari: manual installation instructions with icons
- Add dismissal logic with 7-day localStorage persistence
- Update documentation to reflect 90% project completion

PWA implementation focuses on installability and static asset caching
while avoiding offline API cache (WebRTC requires active connection).

frontend/public/splash/iphone-x.svg

@@ -0,0 +1,9 @@
+<svg width="1125" height="2436" xmlns="http://www.w3.org/2000/svg">
+  <rect width="1125" height="2436" fill="#6366f1"/>
+  <g transform="translate(562.5, 1218)">
+    <circle r="120" fill="#ffffff" opacity="0.1"/>
+    <circle r="80" fill="#ffffff" opacity="0.2"/>
+    <circle r="40" fill="#ffffff"/>
+    <text x="0" y="150" font-family="Arial, sans-serif" font-size="36" font-weight="bold" fill="#ffffff" text-anchor="middle">spotlight.cam</text>
+  </g>
+</svg>