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/lib/api/settings.ts

@@ -223,6 +223,10 @@ export interface BackupEntry {
 }
 
 export const backupsApi = {
+  async createDbBackup(): Promise<string> {
+    return await invoke("create_db_backup");
+  },
+
   async listDbBackups(): Promise<BackupEntry[]> {
     return await invoke("list_db_backups");
   },