feat: PWA install prompt, brand-colored channel icons, image load fix, semantic connection indicator

- Fix image display bug: remove loading='lazy' + use opacity instead of hidden (#66)
- PWA: add id/scope to manifest, fix SW cache name, add install prompt hook + button (#64)
- Colorize channel icons with brand colors (Discord blurple, Telegram blue, etc.) (#68)
- Use semantic green for connection indicator instead of theme accent (#70)
- Replace Sparkles icon with PinchChat logo in sidebar (#65)
- Replace decorative dots in sidebar footer with GitHub link (#71)

public/sw.js

@@ -1,5 +1,5 @@
 // PinchChat Service Worker — cache static assets for offline/instant load
-const CACHE_NAME = 'pinchchat-__SW_VERSION__';
+const CACHE_NAME = 'pinchchat-v1';
 
 // Cache static assets on install
 self.addEventListener('install', (event) => {