feat(P5-4): animated chat backgrounds + pause toggle

5 new CSS-only animated backgrounds in the chat background picker:
- Digital Rain: two-layer vertical stripe scroll with parallax (wide
  stripes at 8s, narrow at 4s via single keyframe with split positions)
- Star Drift: three-layer radial-gradient star field drifting diagonally
- Grid Pulse: neon grid lines that expand/contract (backgroundSize keyframe)
- Aurora Flow: large radial gradient bands sweeping across 200% canvas
- Fireflies: three layers of glowing dots drifting across the viewport

All use vanilla-extract keyframes (GPU-composited transforms/positions,
no canvas, no JS timers). prefers-reduced-motion is respected in
getChatBg() by stripping the animation property at call time. A "Pause
Background Animations" toggle in Settings → Appearance provides an
in-app override for the same purpose.

BG labels de-duplicated ("Digital Rain", "Star Drift", "Aurora Flow")
to avoid the duplicate "Stars" and "Aurora" entries that had appeared.
LIGHT anim-fireflies background corrected from near-black #0a0a10 to
warm white #fffdf0. Four unused keyframe exports removed from
Animations.css.ts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/features/room/RoomView.tsx

@@ -61,6 +61,7 @@ export function RoomView({ eventId }: { eventId?: string }) {
   const roomViewRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
   const [chatBackground] = useSetting(settingsAtom, 'chatBackground');
   const [lotusTerminal] = useSetting(settingsAtom, 'lotusTerminal');
+  const [pauseAnimations] = useSetting(settingsAtom, 'pauseAnimations');
   const theme = useTheme();
   const isDark = theme.kind === ThemeKind.Dark;
 
@@ -98,8 +99,12 @@ export function RoomView({ eventId }: { eventId?: string }) {
 
   const chatBgStyle = useMemo(
     () =>
-      getChatBg(lotusTerminal && chatBackground === 'none' ? 'tactical' : chatBackground, isDark),
-    [chatBackground, lotusTerminal, isDark],
+      getChatBg(
+        lotusTerminal && chatBackground === 'none' ? 'tactical' : chatBackground,
+        isDark,
+        pauseAnimations,
+      ),
+    [chatBackground, lotusTerminal, isDark, pauseAnimations],
   );
 
   return (