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/components/TypingIndicator.tsx

@@ -20,7 +20,7 @@ export function TypingIndicator({ typingUserIds, members }: Props) {
       : t('app:chats.typing_many', { count: typingUserIds.length });
 
   return (
-    <div className="px-6 pb-1 pt-0 text-xs text-neutral-400">
+    <div className="bg-surface-3 px-6 pb-1 pt-0 text-xs text-fg-muted">
       <span className="inline-flex items-center gap-2">
         <TypingDots />
         <span>{text}</span>
@@ -32,9 +32,9 @@ export function TypingIndicator({ typingUserIds, members }: Props) {
 function TypingDots() {
   return (
     <span aria-hidden="true" className="inline-flex items-center gap-0.5">
-      <span className="h-1 w-1 rounded-full bg-neutral-500 [animation:pulse_1.2s_ease-in-out_infinite]" />
-      <span className="h-1 w-1 rounded-full bg-neutral-500 [animation:pulse_1.2s_ease-in-out_infinite] [animation-delay:0.15s]" />
-      <span className="h-1 w-1 rounded-full bg-neutral-500 [animation:pulse_1.2s_ease-in-out_infinite] [animation-delay:0.3s]" />
+      <span className="h-1 w-1 rounded-full bg-fg-muted [animation:pulse_1.2s_ease-in-out_infinite]" />
+      <span className="h-1 w-1 rounded-full bg-fg-muted [animation:pulse_1.2s_ease-in-out_infinite] [animation-delay:0.15s]" />
+      <span className="h-1 w-1 rounded-full bg-fg-muted [animation:pulse_1.2s_ease-in-out_infinite] [animation-delay:0.3s]" />
     </span>
   );
 }