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/styles/globals.css

@@ -4,11 +4,42 @@
 @tailwind components;
 @tailwind utilities;
 
-@layer base {
-  :root {
-    color-scheme: dark;
-  }
+/* ---------------------------------------------------------------------------
+ * Clean-Rail design tokens.
+ *
+ * `--bg-rgb / --fg-rgb / --accent-rgb` are *space-separated rgb triplets*
+ * (no `rgb(...)` wrapper) so Tailwind's `rgb(var(--x) / <alpha-value>)`
+ * alpha modifier works (e.g. `bg-surface/40`).
+ *
+ * `--line` is a full color value with baked-in alpha — theme decides whether
+ * borders are inked-on-white or white-on-ink.
+ * ------------------------------------------------------------------------- */
 
+:root {
+  --bg-rgb: 250 250 249;
+  --bg-2-rgb: 244 244 242;
+  --bg-3-rgb: 255 255 255;
+  --fg-rgb: 10 10 15;
+  --fg-muted-rgb: 115 115 128;
+  --accent-rgb: 79 70 229;
+  --accent-fg-rgb: 255 255 255;
+  --line: rgba(10, 10, 15, 0.08);
+  color-scheme: light;
+}
+
+.dark {
+  --bg-rgb: 5 5 7;
+  --bg-2-rgb: 17 17 24;
+  --bg-3-rgb: 10 10 15;
+  --fg-rgb: 244 244 245;
+  --fg-muted-rgb: 138 138 153;
+  --accent-rgb: 109 115 255;
+  --accent-fg-rgb: 255 255 255;
+  --line: rgba(255, 255, 255, 0.07);
+  color-scheme: dark;
+}
+
+@layer base {
   html,
   body,
   #root {
@@ -16,12 +47,12 @@
   }
 
   body {
-    @apply bg-ink-950 text-neutral-100 font-sans antialiased;
+    @apply bg-surface text-fg font-sans antialiased;
     font-feature-settings: 'cv11', 'ss01';
   }
 
   ::selection {
-    @apply bg-brand-500/40 text-white;
+    @apply bg-accent/40 text-white;
   }
 
   @media (prefers-reduced-motion: reduce) {
@@ -46,4 +77,15 @@
     background-size: 32px 32px;
     background-position: -1px -1px;
   }
+  /* Glass pill used for the fullscreen call controls bar. */
+  .glass-pill {
+    background: rgba(10, 10, 15, 0.7);
+    backdrop-filter: blur(20px);
+    border: 1px solid rgba(255, 255, 255, 0.1);
+  }
+  .glass-chip {
+    background: rgba(0, 0, 0, 0.55);
+    backdrop-filter: blur(10px);
+    color: #fff;
+  }
 }