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/seasonSchedule.test.ts

@@ -0,0 +1,65 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+
+import { getActiveSeason, SEASON_SCHEDULE, SEASON_DATE_RANGES } from './seasonSchedule';
+import { SeasonTheme } from './types';
+
+// Date(year, monthIndex0, day)
+const on = (monthIndex0: number, day: number): Date => new Date(2026, monthIndex0, day);
+
+test('each theme activates on a representative day in its window', () => {
+  const cases: Array<[Date, SeasonTheme]> = [
+    [on(11, 31), 'newyear'], // Dec 31
+    [on(0, 1), 'newyear'], // Jan 1
+    [on(0, 25), 'lunar'], // Jan 25
+    [on(1, 3), 'lunar'], // Feb 3
+    [on(1, 12), 'valentines'], // Feb 12
+    [on(2, 16), 'stpatricks'], // Mar 16
+    [on(3, 1), 'aprilfools'], // Apr 1
+    [on(3, 21), 'earthday'], // Apr 21
+    [on(8, 12), 'arcade'], // Sep 12
+    [on(8, 25), 'autumn'], // Sep 25
+    [on(9, 20), 'halloween'], // Oct 20
+    [on(10, 1), 'halloween'], // Nov 1
+    [on(11, 15), 'christmas'], // Dec 15
+  ];
+  for (const [date, expected] of cases) {
+    assert.equal(getActiveSeason(date), expected, `${date.toDateString()} -> ${expected}`);
+  }
+});
+
+test('priority order resolves overlapping windows (Deep Space outranks Autumn)', () => {
+  // Oct 4-10 is inside Autumn's Oct<=14 window too; Deep Space comes first.
+  assert.equal(getActiveSeason(on(9, 5)), 'deepspace'); // Oct 5
+  // Oct 12 is past Deep Space -> falls through to Autumn.
+  assert.equal(getActiveSeason(on(9, 12)), 'autumn');
+});
+
+test('New Year outranks Lunar New Year on Jan 1-2', () => {
+  assert.equal(getActiveSeason(on(0, 1)), 'newyear');
+  // Jan 22+ is past New Year -> Lunar.
+  assert.equal(getActiveSeason(on(0, 22)), 'lunar');
+});
+
+test('returns null on an off-season day', () => {
+  assert.equal(getActiveSeason(on(5, 15)), null); // Jun 15
+  assert.equal(getActiveSeason(on(6, 4)), null); // Jul 4
+});
+
+test('window boundaries are inclusive at both ends', () => {
+  assert.equal(getActiveSeason(on(1, 10)), 'valentines'); // Feb 10 start
+  assert.equal(getActiveSeason(on(1, 15)), 'valentines'); // Feb 15 end
+  assert.equal(getActiveSeason(on(1, 16)), null); // Feb 16 just after
+});
+
+test('SEASON_DATE_RANGES has a label for every scheduled theme', () => {
+  assert.equal(SEASON_SCHEDULE.length, 11);
+  const themes = SEASON_SCHEDULE.map((e) => e.theme);
+  assert.equal(new Set(themes).size, 11); // unique
+  for (const t of themes) {
+    assert.ok(
+      typeof SEASON_DATE_RANGES[t] === 'string' && SEASON_DATE_RANGES[t].length > 0,
+      `missing date range for ${t}`,
+    );
+  }
+});