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/features/room/RoomInput.tsx

@@ -241,7 +241,10 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
     const showFormat = composerToolbarButtons?.showFormat ?? true;
     const showEmoji = composerToolbarButtons?.showEmoji ?? true;
     const showSticker = composerToolbarButtons?.showSticker ?? true;
-    const showGif = composerToolbarButtons?.showGif ?? true;
+    // [Gitea #68] The GIF picker is opt-in (searches go to Giphy); hide the
+    // toolbar button entirely when it's off so it never opens an empty popover.
+    const [gifPickerEnabled] = useSetting(settingsAtom, 'gifPickerEnabled');
+    const showGif = (composerToolbarButtons?.showGif ?? true) && gifPickerEnabled;
     const showLocation = composerToolbarButtons?.showLocation ?? true;
     const showPoll = composerToolbarButtons?.showPoll ?? true;
     const showVoice = composerToolbarButtons?.showVoice ?? true;