fix(seasonal): auto theme re-evaluates over time; auto clears chat background

- The "auto" seasonal theme was computed once at mount, so a long-lived session
  never crossed a season/holiday-window boundary. SeasonalEffect now re-evaluates
  on an hourly ticker (auto mode only) AND refreshes on entering auto — the
  interval only runs while auto, so a stale mount-time timestamp would otherwise
  resurface on a pinned/off → auto switch (the exact frozen-at-mount bug, caught
  in review). The decision is extracted to a pure resolveSeasonTheme(override,
  now) in seasonSchedule.ts (removing an unsafe cast) and unit-tested.

- Selecting seasonal "auto" while a chat background was set was a silent no-op:
  the seasonal picker only cleared the background for a *specific* theme, and the
  overlay is suppressed while a background is set. Now any active seasonal mode
  ("auto" included) clears the background; only "off" leaves it — symmetric with
  the background picker (which sets seasonal "off"). The overlay guard stays as a
  backstop for legacy persisted state.

Bug-hunt findings from LOTUS_TODO. Three review passes (the 2nd caught the
switch-into-auto staleness); +2 unit tests. Gate-green (tsc, eslint, prettier,
922 tests, build).

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

src/app/components/seasonal/SeasonalEffect.tsx

@@ -1,10 +1,10 @@
-import React, { useMemo } from 'react';
+import React, { 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 { getActiveSeason } from './seasonSchedule';
+import { resolveSeasonTheme } from './seasonSchedule';
 import { HalloweenOverlay } from './themes/Halloween';
 import { ChristmasOverlay } from './themes/Christmas';
 import { NewYearOverlay } from './themes/NewYear';
@@ -96,13 +96,25 @@ export function SeasonalPreview({ theme }: { theme: SeasonTheme }) {
 export function SeasonalEffect() {
   const settings = useAtomValue(settingsAtom);
   const reduced = useReducedMotion();
+  const override = settings.seasonalThemeOverride ?? 'auto';
 
-  const theme = useMemo<SeasonTheme | null>(() => {
-    const override = settings.seasonalThemeOverride ?? 'auto';
-    if (override === 'off') return null;
-    if (override === 'auto') return getActiveSeason(new Date());
-    return override as SeasonTheme;
-  }, [settings.seasonalThemeOverride]);
+  // In auto mode, re-evaluate hourly so a long-lived session crosses a
+  // season/holiday-window boundary (e.g. into a new day) without a reload —
+  // otherwise the active season is frozen at the value it had on mount.
+  const [now, setNow] = useState(() => Date.now());
+  useEffect(() => {
+    if (override !== 'auto') return undefined;
+    // Refresh on entering auto too: `now` may be a stale mount-time value if we
+    // were previously in a pinned/off mode (the interval only runs while auto).
+    setNow(Date.now());
+    const id = window.setInterval(() => setNow(Date.now()), 60 * 60 * 1000);
+    return () => window.clearInterval(id);
+  }, [override]);
+
+  const theme = useMemo<SeasonTheme | null>(
+    () => resolveSeasonTheme(override, now),
+    [override, now],
+  );
 
   if (!theme) return null;
   // Suppress seasonal overlay when a chat background is active — both running simultaneously

src/app/components/seasonal/seasonSchedule.test.ts

@@ -1,7 +1,12 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
 
-import { getActiveSeason, SEASON_SCHEDULE, SEASON_DATE_RANGES } from './seasonSchedule';
+import {
+  getActiveSeason,
+  resolveSeasonTheme,
+  SEASON_SCHEDULE,
+  SEASON_DATE_RANGES,
+} from './seasonSchedule';
 import { SeasonTheme } from './types';
 
 // Date(year, monthIndex0, day)
@@ -52,6 +57,27 @@ test('window boundaries are inclusive at both ends', () => {
   assert.equal(getActiveSeason(on(1, 16)), null); // Feb 16 just after
 });
 
+test('resolveSeasonTheme: off → null, pinned → that theme, auto → active season', () => {
+  const halloweenTs = on(9, 20).getTime(); // Oct 20 → halloween season
+  const offSeasonTs = on(5, 15).getTime(); // Jun 15 → no season
+  // 'off' never renders, regardless of date.
+  assert.equal(resolveSeasonTheme('off', halloweenTs), null);
+  // A pinned theme renders regardless of date (even off-season).
+  assert.equal(resolveSeasonTheme('christmas', offSeasonTs), 'christmas');
+  // 'auto' tracks the active season for the given instant.
+  assert.equal(resolveSeasonTheme('auto', halloweenTs), 'halloween');
+  assert.equal(resolveSeasonTheme('auto', offSeasonTs), null);
+});
+
+test('resolveSeasonTheme: auto re-evaluates as `now` advances across a boundary', () => {
+  // The same 'auto' override yields different themes at different instants — this
+  // is what the SeasonalEffect ticker relies on (incl. the switch-into-auto case
+  // where `now` must be current, not a stale mount value).
+  assert.equal(resolveSeasonTheme('auto', on(9, 20).getTime()), 'halloween'); // Oct 20
+  assert.equal(resolveSeasonTheme('auto', on(11, 15).getTime()), 'christmas'); // Dec 15
+  assert.equal(resolveSeasonTheme('auto', on(6, 4).getTime()), null); // Jul 4
+});
+
 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);

src/app/components/seasonal/seasonSchedule.ts

@@ -93,3 +93,18 @@ export function getActiveSeason(now: Date): SeasonTheme | null {
   const day = now.getDate();
   return SEASON_SCHEDULE.find((entry) => entry.matches(month, day))?.theme ?? null;
 }
+
+/** A seasonal-theme setting value: the active season, a pinned theme, or off. */
+export type SeasonalOverride = SeasonTheme | 'auto' | 'off';
+
+/**
+ * The theme to render for a `seasonalThemeOverride` at time `now` (epoch ms):
+ * 'off' → none, 'auto' → the active season for that instant, else the pinned
+ * theme. Kept pure (and unit-tested) so the decision is verifiable without
+ * mounting the React overlay.
+ */
+export function resolveSeasonTheme(override: SeasonalOverride, now: number): SeasonTheme | null {
+  if (override === 'off') return null;
+  if (override === 'auto') return getActiveSeason(new Date(now));
+  return override;
+}

src/app/features/settings/general/General.tsx

@@ -549,7 +549,11 @@ function Appearance() {
             value={seasonalThemeOverride ?? 'auto'}
             onChange={(v) => {
               setSeasonalThemeOverride(v);
-              if (v !== 'auto' && v !== 'off') setChatBackground('none');
+              // Any active seasonal mode (incl. "auto") is mutually exclusive
+              // with a chat background — else picking it is a silent no-op, since
+              // SeasonalEffect suppresses the overlay while a background is set.
+              // Only "off" leaves the background alone.
+              if (v !== 'off') setChatBackground('none');
             }}
           />
         </Box>