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/Avatar.tsx

@@ -0,0 +1,45 @@
+// Reusable avatar that prefers an uploaded image and falls back to a coloured
+// letter circle. Use this everywhere the app needs to render a profile.
+
+interface Props {
+  url?: string | null | undefined;
+  displayName?: string | null | undefined;
+  className?: string;
+  // Background gradient classes used when no image is set. Defaults to a
+  // brand-tinted fallback; callers can override (e.g. to colour-by-id).
+  fallbackClass?: string;
+  alt?: string;
+}
+
+export function Avatar({
+  url,
+  displayName,
+  className = 'h-10 w-10',
+  fallbackClass = 'bg-accent/20 text-accent',
+  alt,
+}: Props) {
+  if (url) {
+    return (
+      <img
+        src={url}
+        alt={alt ?? displayName ?? ''}
+        className={'shrink-0 rounded-full object-cover ' + className}
+        draggable={false}
+      />
+    );
+  }
+  const letter = (displayName ?? '?').trim().charAt(0).toUpperCase() || '?';
+  return (
+    <div
+      aria-hidden={alt ? undefined : 'true'}
+      className={
+        'flex shrink-0 items-center justify-center rounded-full font-semibold ' +
+        fallbackClass +
+        ' ' +
+        className
+      }
+    >
+      {letter}
+    </div>
+  );
+}