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/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);