fix(settings): enforce authoritative global image-gen policy and non-admin preference persistence

- Enforce authoritative administrator global image_gen_enabled disable in get_user_setting, do_generate_image, and image_editing_provider_available, preventing non-admin user preference overrides while preserving user-disabled opt-outs and can_generate_images privilege checks across direct, gallery, and editor paths.
- Filter non-admin preference persistence in saveImagePreferences to user preference keys only (image_model, image_edit_model, image_quality), preventing global policy toggles from leaking into user preferences.
- Return accurate 200 OK Response from saveImagePreferences after fallback persistence succeeds so settings UI displays 'Saved' instead of false 403 error, while retaining visibility of real errors.
- Load per-user preferences via readImagePreferences in initImageSettings so non-admin selections persist and display correctly after page reload.
- Clear per-user model preferences on successful admin save (POST /api/auth/settings) via PUT /api/prefs/{key} with value: null to prevent stale per-user preferences from shadowing updated global settings.
- Support clearing keys in PUT /api/prefs/{key} when value is null.
- Add comprehensive Python regression tests for global-disabled/user-enabled, global-enabled/user-disabled, and authorized generation, plus Node and browser regression tests for non-admin persistence, toggle isolation, reload, and admin clearing.

routes/prefs_routes.py

@@ -118,8 +118,12 @@ def setup_prefs_routes():
     async def set_pref(request: Request, key: str, body: dict):
         user = get_current_user(request)
         prefs = _load_for_user(user)
-        prefs[key] = body.get("value")
+        val = body.get("value")
+        if val is None:
+            prefs.pop(key, None)
+        else:
+            prefs[key] = val
         _save_for_user(user, prefs)
-        return {"key": key, "value": prefs[key]}
+        return {"key": key, "value": prefs.get(key)}
 
     return router