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/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>