feat: redesign + avatars + theme + call presence fixes (v0.6.0)

Visual:
- Light/dark theme via ThemeContext + CSS vars
- New design tokens (surface/fg/accent/line/etc.) across all pages
- Reusable Avatar component (img + letter fallback) wired into UserBar,
  ConversationHeader, ChatsPage, FriendsPage, GroupInfoPanel, IncomingCallPanel

Calls:
- Split call UI: IncomingCallPanel, InCallPanel, CallControls,
  CallParticipantTile, ScreenShareViewer
- Active speaker hook (useActiveSpeakers)
- Fix: ActiveCallBanner stayed hidden after hangup while peers in room.
  - useCallPresence: bind presence callbacks only when we own subscribe
    (Supabase forbids .on() after .subscribe() on shared dedup'd channels)
  - useCallPresence: never removeChannel — channel is shared with CallContext
    so tearing it down on ConversationHeader unmount killed live tracking
  - ActiveCallBanner: lastCallConversationId fallback so banner shows
    instantly after hangup, auto-dismiss when room confirmed empty
- Drop unused useAnyActiveCall

Bump tauri version 0.5.0 -> 0.6.0

apps/desktop/src/App.tsx

@@ -7,6 +7,7 @@ import { AuthProvider } from './context/AuthContext';
 import { CallProvider } from './context/CallContext';
 import { ConversationsProvider } from './context/ConversationsContext';
 import { FriendshipsProvider } from './context/FriendshipsContext';
+import { ThemeProvider } from './context/ThemeContext';
 import { AdminPage } from './pages/AdminPage';
 import { AuthCallbackPage } from './pages/AuthCallbackPage';
 import { AuthPage } from './pages/AuthPage';
@@ -18,6 +19,7 @@ import { SettingsPage } from './pages/SettingsPage';
 
 export function App() {
   return (
+    <ThemeProvider>
     <AuthProvider>
       <FriendshipsProvider>
         <ConversationsProvider>
@@ -51,5 +53,6 @@ export function App() {
         </ConversationsProvider>
       </FriendshipsProvider>
     </AuthProvider>
+    </ThemeProvider>
   );
 }