fix: treat missing db file as error in manual backup to prevent false success toast

backup_database_file() returning Ok(None) was silently resolved as null
on the frontend, bypassing try/catch and showing a success toast without
actually creating a backup file. Now None is converted to an explicit
Err so the frontend correctly displays an error toast.

src-tauri/src/commands/import_export.rs

@@ -123,6 +123,24 @@ pub async fn open_zip_file_dialog<R: tauri::Runtime>(
 
 // ─── Database backup management ─────────────────────────────
 
+/// Manually create a database backup
+#[tauri::command]
+pub async fn create_db_backup(state: State<'_, AppState>) -> Result<String, String> {
+    let db = state.db.clone();
+    tauri::async_runtime::spawn_blocking(move || match db.backup_database_file()? {
+        Some(path) => Ok(path
+            .file_name()
+            .map(|f| f.to_string_lossy().into_owned())
+            .unwrap_or_default()),
+        None => Err(AppError::Config(
+            "Database file not found, backup skipped".to_string(),
+        )),
+    })
+    .await
+    .map_err(|e| format!("Backup failed: {e}"))?
+    .map_err(|e: AppError| e.to_string())
+}
+
 /// List all database backup files
 #[tauri::command]
 pub fn list_db_backups() -> Result<Vec<BackupEntry>, String> {