feat(seasonal): Autumn pass 2 — no tint, a gust, a real preview (#237)

Draft for human review (#237 must be approved and closed by a human).

- Removed the full-screen amber wash, sun-shaft bands and vignette: they
  turned the user's theme beige (muddy brown on dark) and lowered contrast
  everywhere. The leaves carry the season on their own. Also removed the
  dust motes (invisible specks) and the now-unused keyframes.
- Gust: every 42 s (first ~8 s after load) five leaves blow across the screen
  together, rising and spinning, in ~5 s. Linear timing — an eased curve is
  applied per keyframe segment and made them hang mid-air for a second.
- Softer flutter (leaves flattened into slivers mid-flip), stronger leaf
  colour (opacity 0.58–0.73, less pale gold tip), brighter on dark themes.
- Reduced motion on phones: no icon rail there, so the resting leaves tuck
  further off-edge and smaller instead of landing on the timeline icons.
- Settings swatch: new optional `preview` prop on seasonal overlays; Autumn
  draws a warm backdrop with arranged leaves (the reduced scene rendered in
  the tile as a black square with two edge fragments).

Rendered and inspected: light/dark/phone at 10/15/20 s, reduced on desktop
and phone, gust tracked frame by frame, the Settings swatch; 60 fps with the
theme on vs off (p95 frame 16.7 ms both).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/seasonal/SeasonalEffect.tsx

@@ -3,13 +3,13 @@ import { useAtomValue } from 'jotai';
 import { settingsAtom } from '../../state/settings';
 import { useReducedMotion } from '../../hooks/useReducedMotion';
 import { zIndices } from '../../styles/zIndex';
-import { SeasonTheme } from './types';
+import { SeasonTheme, SeasonalOverlayProps } from './types';
 import { resolveSeasonTheme } from './seasonSchedule';
 
 // [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 }> }>) =>
+const lazyOverlay = (load: () => Promise<{ default: React.ComponentType<SeasonalOverlayProps> }>) =>
   React.lazy(load);
 const HalloweenOverlay = lazyOverlay(() =>
   import('./themes/Halloween').then((m) => ({ default: m.HalloweenOverlay })),
@@ -52,7 +52,11 @@ export type { SeasonTheme };
 
 // ─── Overlay content map (shared between SeasonalOverlay and SeasonalPreview) ──
 
-function buildOverlayContent(theme: SeasonTheme, reduced: boolean): React.ReactNode {
+function buildOverlayContent(
+  theme: SeasonTheme,
+  reduced: boolean,
+  preview = false,
+): React.ReactNode {
   switch (theme) {
     case 'halloween':
       return <HalloweenOverlay reduced={reduced} />;
@@ -61,7 +65,7 @@ function buildOverlayContent(theme: SeasonTheme, reduced: boolean): React.ReactN
     case 'newyear':
       return <NewYearOverlay reduced={reduced} />;
     case 'autumn':
-      return <AutumnOverlay reduced={reduced} />;
+      return <AutumnOverlay reduced={reduced} preview={preview} />;
     case 'aprilfools':
       return <AprilFoolsOverlay reduced={reduced} />;
     case 'lunar':
@@ -122,7 +126,7 @@ export function SeasonalPreview({ theme }: { theme: SeasonTheme }) {
         containerType: 'inline-size',
       }}
     >
-      <Suspense fallback={null}>{buildOverlayContent(theme, true)}</Suspense>
+      <Suspense fallback={null}>{buildOverlayContent(theme, true, true)}</Suspense>
     </div>
   );
 }