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/settings/general/General.tsx

@@ -331,6 +331,7 @@ function Appearance() {
     settingsAtom,
     'glassmorphismSidebar',
   );
+  const [pauseAnimations, setPauseAnimations] = useSetting(settingsAtom, 'pauseAnimations');
 
   return (
     <Box direction="Column" gap="100">
@@ -390,6 +391,14 @@ function Appearance() {
         </Box>
       </SequenceCard>
 
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Pause Background Animations"
+          description="Stop background animation to reduce motion or improve performance."
+          after={<Switch variant="Primary" value={pauseAnimations} onChange={setPauseAnimations} />}
+        />
+      </SequenceCard>
+
       <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
         <SettingTile
           title="Show Profile on Every Message"
@@ -1056,6 +1065,7 @@ function Calls() {
 
 function ChatBgGrid() {
   const [chatBackground, setChatBackground] = useSetting(settingsAtom, 'chatBackground');
+  const [pauseAnimations] = useSetting(settingsAtom, 'pauseAnimations');
   const theme = useTheme();
   const isDark = theme.kind === ThemeKind.Dark;
 
@@ -1080,7 +1090,7 @@ function ChatBgGrid() {
                   : '2px solid rgba(128,128,128,0.25)',
               padding: 0,
               overflow: 'hidden',
-              ...getChatBg(opt.value as ChatBackground, isDark),
+              ...getChatBg(opt.value as ChatBackground, isDark, pauseAnimations),
             }}
           />
           <Text