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.

static/js/settings/imageModels.js

@@ -4,7 +4,7 @@
 // them from the same places, so load order and init sequence are untouched.
 
 import { byId as el } from './dom.js';
-import { saveImagePreferences as _postSettings } from './imagePreferences.js';
+import { saveImagePreferences as _postSettings, readImagePreferences } from './imagePreferences.js';
 import { modelCaps } from '../editor/ai-models.js';
 import { sortModelIds } from '../modelSort.js';
 let preferencesListener = null;
@@ -46,8 +46,7 @@ export async function initImageSettings() {
     }
   } catch (e) { console.warn('Failed to load models for image settings', e); }
   try {
-    const settingsRes = await fetch('/api/auth/settings', { credentials: 'same-origin' });
-    const settings = await settingsRes.json();
+    const settings = await readImagePreferences();
     if (settings.image_model) modelSel.value = settings.image_model;
     if (editModelSel) editModelSel.value = settings.image_edit_model || '';
     if (settings.image_quality) qualSel.value = settings.image_quality;