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)

src/hooks/usePwaInstall.ts

@@ -0,0 +1,52 @@
+import { useState, useEffect, useCallback, useRef } from 'react';
+
+interface BeforeInstallPromptEvent extends Event {
+  prompt(): Promise<void>;
+  userChoice: Promise<{ outcome: 'accepted' | 'dismissed' }>;
+}
+
+// Check standalone mode outside of React to avoid setState-in-effect
+const isStandalone = typeof window !== 'undefined' && window.matchMedia('(display-mode: standalone)').matches;
+
+export function usePwaInstall() {
+  const [deferredPrompt, setDeferredPrompt] = useState<BeforeInstallPromptEvent | null>(null);
+  const [isInstalled, setIsInstalled] = useState(isStandalone);
+  const installedRef = useRef(isStandalone);
+
+  useEffect(() => {
+    if (installedRef.current) return;
+
+    const handler = (e: Event) => {
+      e.preventDefault();
+      setDeferredPrompt(e as BeforeInstallPromptEvent);
+    };
+
+    const installedHandler = () => {
+      installedRef.current = true;
+      setIsInstalled(true);
+      setDeferredPrompt(null);
+    };
+
+    window.addEventListener('beforeinstallprompt', handler);
+    window.addEventListener('appinstalled', installedHandler);
+
+    return () => {
+      window.removeEventListener('beforeinstallprompt', handler);
+      window.removeEventListener('appinstalled', installedHandler);
+    };
+  }, []);
+
+  const install = useCallback(async () => {
+    if (!deferredPrompt) return false;
+    await deferredPrompt.prompt();
+    const { outcome } = await deferredPrompt.userChoice;
+    setDeferredPrompt(null);
+    return outcome === 'accepted';
+  }, [deferredPrompt]);
+
+  return {
+    canInstall: !!deferredPrompt && !isInstalled,
+    isInstalled,
+    install,
+  };
+}