fix(desktop): swap GIF provider from Tenor (closed Jan 2026) to GIPHY

Google closed Tenor v2 to new API clients in Jan 2026, so the only
people who could use the picker were those with a pre-existing Google
Cloud Console key. Swapped to GIPHY's Developer API (still open, free
keys at https://developers.giphy.com/dashboard/).

- Env var renamed VITE_TENOR_API_KEY → VITE_GIPHY_API_KEY
- Endpoint, response mapping, error sentinel updated
- File still named tenor.ts for import-path stability — renaming
  later if it bothers anyone
- Public GifResult interface unchanged so the picker UI didn't need
  edits beyond the error-message switch

apps/desktop/src/components/GifPicker.tsx

@@ -99,8 +99,8 @@ export function GifPicker({ open, onClose, onPick }: Props) {
         )}
         {error && (
           <p className="col-span-3 px-2 py-4 text-center text-xs text-rose-300">
-            {error === 'tenor_api_key_missing'
-              ? 'GIFs sind nicht konfiguriert (VITE_TENOR_API_KEY fehlt).'
+            {error === 'giphy_api_key_missing'
+              ? 'GIFs sind nicht konfiguriert (VITE_GIPHY_API_KEY fehlt).'
               : 'GIFs gerade nicht verfügbar.'}
           </p>
         )}