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/components/GifPicker.tsx

@@ -209,6 +209,13 @@ type GifPickerProps = {
 
 export function GifPicker({ apiKey, onSelect, requestClose }: GifPickerProps) {
   const [lotusTerminal] = useSetting(settingsAtom, 'lotusTerminal');
+  const [gifPickerEnabled] = useSetting(settingsAtom, 'gifPickerEnabled');
+
+  // Searches (and every keystroke) go straight to Giphy's API, so the picker
+  // is opt-in (Settings → Messages) and must not render or fetch until then.
+  if (!gifPickerEnabled) {
+    return null;
+  }
 
   const containerStyle = lotusTerminal
     ? {