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/pages/App.tsx

@@ -16,6 +16,7 @@ import { useCompositionEndTracking } from '../hooks/useComposingCheck';
 import { settingsAtom } from '../state/settings';
 import { LotusToastContainer } from '../features/toast/LotusToastContainer';
 import { useTauriNotificationBadge } from '../hooks/useTauriNotificationBadge';
+import { SeasonalEffect } from '../components/seasonal/SeasonalEffect';
 
 const FONT_MAP: Record<string, string> = {
   system: "system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif",
@@ -139,6 +140,7 @@ function App() {
                           <AppearanceEffects />
                           <TauriEffects />
                           <RouterProvider router={createRouter(clientConfig, screenSize)} />
+                          <SeasonalEffect />
                           <NightLightOverlay />
                           <LotusToastContainer />
                         </JotaiProvider>