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/lib/useActiveSpeakers.ts

@@ -0,0 +1,35 @@
+import type { Participant, Room } from 'livekit-client';
+import { RoomEvent } from 'livekit-client';
+import { useEffect, useState } from 'react';
+
+// Subscribe to LiveKit ActiveSpeakersChanged to drive the speaking-ring pulse
+// on participant tiles. Returns the set of currently speaking identities
+// (includes local participant when they're active).
+export function useActiveSpeakers(room: Room | null): Set<string> {
+  const [ids, setIds] = useState<Set<string>>(() => new Set());
+
+  useEffect(() => {
+    if (!room) {
+      setIds(new Set());
+      return;
+    }
+
+    const update = (speakers: Participant[]) => {
+      const next = new Set<string>();
+      for (const s of speakers) {
+        if (s.identity) next.add(s.identity);
+      }
+      setIds(next);
+    };
+
+    // Seed from current state.
+    update(room.activeSpeakers ?? []);
+
+    room.on(RoomEvent.ActiveSpeakersChanged, update);
+    return () => {
+      room.off(RoomEvent.ActiveSpeakersChanged, update);
+    };
+  }, [room]);
+
+  return ids;
+}