perf: lazy-load seasonal overlays as their own chunks (#167)

All 11 seasonal overlays (particles + vanilla-extract keyframes) were in the
main JS and CSS for everyone year-round. Each is now a React.lazy chunk fetched
only when that season is active (auto date or override) or previewed in
Settings → Appearance; the schedule/override decision stays in the main bundle.

check-bundle-size: largest chunk 332.4 kB → 320.1 kB gzip; main CSS 68.4 kB →
56.5 kB; total gzip 1551.6 kB → 1561.3 kB (per-chunk overhead, only paid when
a season is on). Verified headless: no theme chunk requested at startup with
'auto' out of season; the Settings grid fetches the 11 previews; picking
Halloween renders the full-screen overlay (20 particle nodes).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/seasonal/SeasonalEffect.tsx

@@ -1,21 +1,49 @@
-import React, { useEffect, useMemo, useState } from 'react';
+import React, { Suspense, useEffect, useMemo, useState } from 'react';
 import { useAtomValue } from 'jotai';
 import { settingsAtom } from '../../state/settings';
 import { useReducedMotion } from '../../hooks/useReducedMotion';
 import { zIndices } from '../../styles/zIndex';
 import { SeasonTheme } from './types';
 import { resolveSeasonTheme } from './seasonSchedule';
-import { HalloweenOverlay } from './themes/Halloween';
-import { ChristmasOverlay } from './themes/Christmas';
-import { NewYearOverlay } from './themes/NewYear';
-import { AutumnOverlay } from './themes/Autumn';
-import { AprilFoolsOverlay } from './themes/AprilFools';
-import { LunarNewYearOverlay } from './themes/LunarNewYear';
-import { ValentinesOverlay } from './themes/Valentines';
-import { StPatricksOverlay } from './themes/StPatricks';
-import { EarthDayOverlay } from './themes/EarthDay';
-import { DeepSpaceOverlay } from './themes/DeepSpace';
-import { ArcadeOverlay } from './themes/Arcade';
+
+// [Gitea #167] Each overlay (its particles + keyframes) is its own chunk,
+// fetched only when that season is active or previewed in Settings — the
+// date/override decision stays in the main bundle.
+const lazyOverlay = (load: () => Promise<{ default: React.ComponentType<{ reduced: boolean }> }>) =>
+  React.lazy(load);
+const HalloweenOverlay = lazyOverlay(() =>
+  import('./themes/Halloween').then((m) => ({ default: m.HalloweenOverlay })),
+);
+const ChristmasOverlay = lazyOverlay(() =>
+  import('./themes/Christmas').then((m) => ({ default: m.ChristmasOverlay })),
+);
+const NewYearOverlay = lazyOverlay(() =>
+  import('./themes/NewYear').then((m) => ({ default: m.NewYearOverlay })),
+);
+const AutumnOverlay = lazyOverlay(() =>
+  import('./themes/Autumn').then((m) => ({ default: m.AutumnOverlay })),
+);
+const AprilFoolsOverlay = lazyOverlay(() =>
+  import('./themes/AprilFools').then((m) => ({ default: m.AprilFoolsOverlay })),
+);
+const LunarNewYearOverlay = lazyOverlay(() =>
+  import('./themes/LunarNewYear').then((m) => ({ default: m.LunarNewYearOverlay })),
+);
+const ValentinesOverlay = lazyOverlay(() =>
+  import('./themes/Valentines').then((m) => ({ default: m.ValentinesOverlay })),
+);
+const StPatricksOverlay = lazyOverlay(() =>
+  import('./themes/StPatricks').then((m) => ({ default: m.StPatricksOverlay })),
+);
+const EarthDayOverlay = lazyOverlay(() =>
+  import('./themes/EarthDay').then((m) => ({ default: m.EarthDayOverlay })),
+);
+const DeepSpaceOverlay = lazyOverlay(() =>
+  import('./themes/DeepSpace').then((m) => ({ default: m.DeepSpaceOverlay })),
+);
+const ArcadeOverlay = lazyOverlay(() =>
+  import('./themes/Arcade').then((m) => ({ default: m.ArcadeOverlay })),
+);
 
 // SeasonTheme + the date-window logic now live in leaf modules (single source
 // of truth, shared with the settings UI). Re-exported here for existing
@@ -69,7 +97,7 @@ function SeasonalOverlay({ theme, reduced }: { theme: SeasonTheme; reduced: bool
         overflow: 'hidden',
       }}
     >
-      {buildOverlayContent(theme, reduced)}
+      <Suspense fallback={null}>{buildOverlayContent(theme, reduced)}</Suspense>
     </div>
   );
 }
@@ -94,7 +122,7 @@ export function SeasonalPreview({ theme }: { theme: SeasonTheme }) {
         containerType: 'inline-size',
       }}
     >
-      {buildOverlayContent(theme, true)}
+      <Suspense fallback={null}>{buildOverlayContent(theme, true)}</Suspense>
     </div>
   );
 }