feat: Implement core Python FastAPI backend for user authentication, vault management, and file operations.

backend/vault_settings.py

@@ -64,9 +64,12 @@ def save_vault_settings() -> None:
             _SETTINGS_PATH.write_text(content, encoding="utf-8")
             logger.info(f"Successfully saved settings for {len(_vault_settings)} vaults to {_SETTINGS_PATH}")
         except PermissionError as e:
-            logger.error(f"Permission denied writing to {_SETTINGS_PATH}: {e}")
-            logger.error(f"Check that user has write permissions to {_SETTINGS_PATH.parent}")
-            raise
+            logger.critical("=" * 60)
+            logger.critical(f"ERREUR DE PERMISSION : Impossible d'écrire dans {_SETTINGS_PATH.parent}")
+            logger.critical("Le conteneur n'a pas les droits sur le volume monté dans /app/data.")
+            logger.critical("FIX : Exécutez sur l'hôte (Linux) : sudo chown -R 1000:1000 /votre/chemin/data")
+            logger.critical("=" * 60)
+            raise e
         except Exception as e:
             logger.error(f"Failed to save vault settings to {_SETTINGS_PATH}: {e}")
             logger.error(f"Error type: {type(e).__name__}")