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