Translate admin pages - Create admin.json files and translate all admin pages

- Create admin.json translation files for es, en, ca with all admin strings
- Update IntlProvider to include admin namespace
- Translate admin/invites/page.tsx - all strings now use translations
- Translate admin/trades/page.tsx - all strings now use translations
- Translate admin/price-history/page.tsx - all strings now use translations
- Translate admin/availability/page.tsx - all strings now use translations
- Add 'saving' key to common.json for all languages
- Fix linting errors: add t to useCallback dependencies
- All admin pages now fully multilingual

frontend/locales/es/common.json

@@ -2,6 +2,7 @@
   "loading": "Cargando...",
   "error": "Error",
   "save": "Guardar",
+  "saving": "Guardando...",
   "cancel": "Cancelar",
   "back": "Atrás",
   "continue": "Continuar",