Add UI settings API and improve context initialization

- Add /settings/ui GET and PUT endpoints for UI settings
- Improve ThemeContext with better initialization and auto accent handling
- Update SidebarContext with expanded state persistence
- Fix context initialization in ModulesContext and ViewModeContext
- Update components to use improved theme/sidebar contexts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/api/v1/settings.py

@@ -10,6 +10,7 @@ from app.models.user import User
 from app.core.settings_registry import (
     THEME_KEYS,
     MODULE_KEYS,
+    UI_KEYS,
     SETTINGS_REGISTRY,
     get_default_value,
 )
@@ -82,6 +83,47 @@ def get_module_settings(
     return result
 
 
+@router.get("/ui", response_model=dict[str, Any])
+def get_ui_settings(
+    *,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user)
+) -> Any:
+    """
+    Get UI settings (accessible to all authenticated users).
+
+    Returns UI/layout settings that apply to all users.
+    """
+    result = {}
+    for key in UI_KEYS:
+        setting = crud.settings.get_setting(db, key=key)
+        if setting:
+            result[key] = setting.get_value()
+        else:
+            result[key] = get_default_value(key)
+    return result
+
+
+@router.put("/ui", response_model=dict[str, Any])
+def update_ui_settings(
+    *,
+    db: Session = Depends(get_db),
+    ui_data: dict[str, Any],
+    current_user: User = Depends(get_current_superuser)
+) -> Any:
+    """
+    Update UI settings (admin only).
+
+    Updates multiple UI/layout settings at once.
+    """
+    result = {}
+    for key, value in ui_data.items():
+        if key in UI_KEYS:
+            setting = crud.settings.update_setting(db, key=key, value=value)
+            result[key] = setting.get_value()
+    return result
+
+
 @router.put("/modules", response_model=dict[str, Any])
 def update_module_settings(
     *,