feat(seasonal): show Auto activation dates in settings + single-source schedule

Settings never told the user which days "Auto" turns each seasonal theme on.
Extracted the date windows out of getActiveSeason into a shared SEASON_SCHEDULE
(seasonSchedule.ts) — the single source of truth for both the runtime Auto
selector and the settings UI, so displayed dates can't drift from real activation.

- seasonal/types.ts: SeasonTheme + SeasonalOverlayProps (leaf module).
- seasonal/seasonSchedule.ts: priority-ordered SEASON_SCHEDULE with human date
  ranges + SEASON_DATE_RANGES + getActiveSeason (behavior-preserving refactor).
- SeasonalEffect.tsx: consume the shared type/selector; re-export SeasonTheme.
- General.tsx: per-theme date caption under each swatch ("Oct 15 – Nov 1"), Auto
  reads "By calendar", and the section description explains it.
- seasonSchedule.test.ts (6): representative day per theme, overlap priority
  (Deep Space > Autumn, New Year > Lunar), inclusive boundaries, off-season null.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/seasonal/SeasonalEffect.tsx

@@ -2,6 +2,8 @@ import React, { useMemo } from 'react';
 import { useAtomValue } from 'jotai';
 import { settingsAtom } from '../../state/settings';
 import { zIndices } from '../../styles/zIndex';
+import { SeasonTheme } from './types';
+import { getActiveSeason } from './seasonSchedule';
 import {
   animSeasonFall,
   animLeafFall,
@@ -16,48 +18,10 @@ import {
   animPixelBlink,
 } from './Seasonal.css';
 
-export type SeasonTheme =
-  | 'halloween'
-  | 'christmas'
-  | 'newyear'
-  | 'autumn'
-  | 'aprilfools'
-  | 'lunar'
-  | 'valentines'
-  | 'stpatricks'
-  | 'earthday'
-  | 'deepspace'
-  | 'arcade';
-
-function getActiveSeason(now: Date): SeasonTheme | null {
-  const m = now.getMonth() + 1; // 1-12
-  const d = now.getDate();
-
-  // New Year takes highest priority (Dec 31 – Jan 2)
-  if ((m === 12 && d === 31) || (m === 1 && d <= 2)) return 'newyear';
-  // Valentine's Day (Feb 10–15)
-  if (m === 2 && d >= 10 && d <= 15) return 'valentines';
-  // St. Patrick's Day (March 15–18)
-  if (m === 3 && d >= 15 && d <= 18) return 'stpatricks';
-  // April Fool's (April 1)
-  if (m === 4 && d === 1) return 'aprilfools';
-  // Earth Day (April 20–23)
-  if (m === 4 && d >= 20 && d <= 23) return 'earthday';
-  // Lunar New Year (Jan 22 – Feb 5, approximate fixed window)
-  if ((m === 1 && d >= 22) || (m === 2 && d <= 5)) return 'lunar';
-  // International Video Game Day (Sept 12)
-  if (m === 9 && d === 12) return 'arcade';
-  // World Space Week (Oct 4–10)
-  if (m === 10 && d >= 4 && d <= 10) return 'deepspace';
-  // Halloween (Oct 15 – Nov 1)
-  if ((m === 10 && d >= 15) || (m === 11 && d === 1)) return 'halloween';
-  // Christmas (Dec 10–30)
-  if (m === 12 && d >= 10) return 'christmas';
-  // Autumn (Sept 21 – Oct 31, excluding Halloween/Deep Space windows above)
-  if ((m === 9 && d >= 21) || (m === 10 && d <= 14)) return 'autumn';
-
-  return null;
-}
+// SeasonTheme + the date-window logic now live in leaf modules (single source
+// of truth, shared with the settings UI). Re-exported here for existing
+// importers that still reach for it from this file.
+export type { SeasonTheme };
 
 // ─── Individual theme overlays ────────────────────────────────────────────────