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/features/settings/general/General.tsx

@@ -57,6 +57,7 @@ import {
   settingsAtom,
 } from '../../../state/settings';
 import { SeasonalPreview, SeasonTheme } from '../../../components/seasonal/SeasonalEffect';
+import { SEASON_DATE_RANGES } from '../../../components/seasonal/seasonSchedule';
 import { SettingTile } from '../../../components/setting-tile';
 import { KeySymbol } from '../../../utils/key-symbol';
 import { isMacOS } from '../../../utils/user-agent';
@@ -438,7 +439,7 @@ function Appearance() {
       >
         <SettingTile
           title="Seasonal Theme"
-          description="Decorative overlays for holidays and events. Preview below — click to select."
+          description="Decorative overlays for holidays and events. “Auto” follows the calendar — each theme below shows the dates it turns on. Click to select."
         />
         <Box style={{ padding: `0 ${config.space.S400} ${config.space.S300}` }}>
           <SeasonalBgGrid
@@ -1699,6 +1700,13 @@ function SeasonalBgGrid({
             <Text size="T200" style={selected ? { color: color.Primary.Main } : undefined}>
               {opt.label}
             </Text>
+            {(opt.value === 'auto' || !isSpecial) && (
+              <Text size="T200" style={{ opacity: 0.6, textAlign: 'center' }}>
+                {opt.value === 'auto'
+                  ? 'By calendar'
+                  : SEASON_DATE_RANGES[opt.value as SeasonTheme]}
+              </Text>
+            )}
           </Box>
         );
       })}