feat: seasonal theme overlays + improved animated chat backgrounds

Adds 11 CSS-only seasonal overlays (Halloween, Christmas, New Year, Autumn,
April Fool's, Lunar New Year, Valentine's Day, St. Patrick's Day, Earth Day,
Deep Space, Retro Arcade) with date-based auto-detection and a manual override
dropdown in Settings → Appearance → Seasonal Theme. All themes respect
prefers-reduced-motion. SeasonalEffect mounts at z-index 9997 in App.tsx.

Also rewrites all 5 animated chat background keyframes for smoother, more
organic motion: Digital Rain gains a phosphor glow flicker; Star Drift now
loops each layer by exactly its own tile size (no more seam); Grid Pulse adds
an independent brightness oscillation at a prime period; Aurora Flow drives
all four gradient layers through distinct paths; Fireflies adds glow-pulse and
opacity-blink animations at prime periods for unsynchronised bioluminescence.

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

src/app/state/settings.ts

@@ -133,6 +133,21 @@ export interface Settings {
   afkTimeoutMinutes: number;
 
   callJoinLeaveSound: 'off' | 'chime' | 'soft' | 'retro';
+
+  seasonalThemeOverride:
+    | 'auto'
+    | 'off'
+    | 'halloween'
+    | 'christmas'
+    | 'newyear'
+    | 'autumn'
+    | 'aprilfools'
+    | 'lunar'
+    | 'valentines'
+    | 'stpatricks'
+    | 'earthday'
+    | 'deepspace'
+    | 'arcade';
 }
 
 const defaultSettings: Settings = {
@@ -208,6 +223,8 @@ const defaultSettings: Settings = {
   afkTimeoutMinutes: 10,
 
   callJoinLeaveSound: 'chime',
+
+  seasonalThemeOverride: 'auto',
 };
 
 export const getSettings = (): Settings => {