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.

src/ai_interaction.py

@@ -1181,9 +1181,9 @@ def image_editing_provider_available(owner: Optional[str] = None) -> bool:
     request on this so it never falls back to deterministic approximations.
     """
     try:
-        from src.settings import load_settings
+        from src.settings import get_user_setting, load_settings
 
-        if not load_settings().get("image_gen_enabled", True):
+        if not load_settings().get("image_gen_enabled", True) or not get_user_setting("image_gen_enabled", owner=owner or "", default=True):
             return False
     except Exception:
         pass
@@ -1283,8 +1283,12 @@ async def do_generate_image(content: str, session_id: Optional[str] = None, owne
         def get_user_setting(key, owner="", default=None):
             return default
 
-    if not get_user_setting("image_gen_enabled", owner=owner, default=True):
+    # The administrator's global image-generation disable switch is authoritative.
+    # Non-admin preferences cannot re-enable generation when disabled globally.
+    if not _settings.get("image_gen_enabled", True):
         return {"error": "Image generation is disabled by the administrator."}
+    if not get_user_setting("image_gen_enabled", owner=owner or "", default=True):
+        return {"error": "Image generation is disabled by user preference."}
 
     # Use user- or admin-configured model/quality if not specified by the tool call
     if not model_spec:

src/settings.py

@@ -314,6 +314,20 @@ def get_user_setting(key: str, owner: str = "", default: Any = None) -> Any:
     Falls back gracefully if the prefs module can't be imported (cycle/early
     boot) — admin-global settings keep working.
     """
+    if key == "image_gen_enabled":
+        global_val = get_setting("image_gen_enabled", default if default is not None else False)
+        if not global_val:
+            return False
+        if owner:
+            try:
+                from routes.prefs_routes import _load_for_user
+                prefs = _load_for_user(owner) or {}
+                if "image_gen_enabled" in prefs and prefs["image_gen_enabled"] is not None:
+                    return bool(prefs["image_gen_enabled"])
+            except Exception:
+                pass
+        return bool(global_val)
+
     if owner and key in _PER_USER_KEYS:
         try:
             from routes.prefs_routes import _load_for_user