feat: add comprehensive error handling and logging for vault settings save operations with permission error detection and detailed error messages in UI toast notifications

backend/main.py

@@ -1477,7 +1477,20 @@ async def api_update_vault_settings(vault_name: str, body: dict = Body(...), cur
         settings_to_update["hiddenWhitelist"] = body["hiddenWhitelist"]
     
     # Update persisted settings
-    updated = update_vault_setting(vault_name, settings_to_update)
+    try:
+        updated = update_vault_setting(vault_name, settings_to_update)
+    except PermissionError as e:
+        logger.error(f"Permission error saving settings for vault '{vault_name}': {e}")
+        raise HTTPException(
+            status_code=500, 
+            detail=f"Permission denied: Cannot write to settings file. Check /app/data permissions."
+        )
+    except Exception as e:
+        logger.error(f"Error saving settings for vault '{vault_name}': {e}")
+        raise HTTPException(
+            status_code=500, 
+            detail=f"Failed to save settings: {str(e)}"
+        )
     
     # Update in-memory vault config
     if vault_name in vault_config: