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/state/settings.ts

@@ -26,7 +26,12 @@ export type ChatBackground =
   | 'hexgrid'
   | 'waves'
   | 'neon'
-  | 'aurora';
+  | 'aurora'
+  | 'anim-rain'
+  | 'anim-stars'
+  | 'anim-pulse'
+  | 'anim-aurora'
+  | 'anim-fireflies';
 export enum MessageLayout {
   Modern = 0,
   Compact = 1,
@@ -94,6 +99,8 @@ export interface Settings {
   deafenKey: string;
 
   warnOnUnverifiedDevices: boolean;
+
+  pauseAnimations: boolean;
 }
 
 const defaultSettings: Settings = {
@@ -157,6 +164,8 @@ const defaultSettings: Settings = {
   deafenKey: 'KeyM',
 
   warnOnUnverifiedDevices: false,
+
+  pauseAnimations: false,
 };
 
 export const getSettings = (): Settings => {