fix(settings): validate seasonalThemeOverride; commit page zoom on blur; GIF picker opt-in

- seasonalThemeOverride is coerced to 'auto' when the persisted value is
  outside the union, like every other enum setting (tested).
- PageZoomInput commits on blur as well as Enter.
- New gifPickerEnabled setting (default off) with a disclosure that GIF
  searches go to Giphy; the picker doesn't mount and the composer button
  is hidden while off.

Fixes #74
Fixes #75
Fixes #68

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/state/settings.test.ts

@@ -56,6 +56,18 @@ test('coerces unknown persisted denoise model / ringtone id back to defaults', (
   assert.equal(valid.ringtoneId, 'chime');
 });
 
+test('coerces unknown persisted seasonalThemeOverride back to auto', () => {
+  setStored(null);
+  const defaults = getSettings();
+  assert.equal(defaults.seasonalThemeOverride, 'auto');
+
+  setStored(JSON.stringify({ seasonalThemeOverride: 'retired-theme' }));
+  assert.equal(getSettings().seasonalThemeOverride, 'auto');
+
+  setStored(JSON.stringify({ seasonalThemeOverride: 'earthday' }));
+  assert.equal(getSettings().seasonalThemeOverride, 'earthday');
+});
+
 test('merges stored values over defaults', () => {
   setStored(JSON.stringify({ callNoiseSuppression: 'off', someUnknownKey: 1 }));
   const s = getSettings();