feat(backup): add pre-migration backup, periodic backup, backfill warning, and backup management UI

Four improvements to the database backup mechanism:

1. Auto backup before schema migration - creates a snapshot when
   upgrading from an older database version, providing a safety net
   beyond the existing SAVEPOINT rollback mechanism.

2. Periodic startup backup - checks on app launch whether the latest
   backup is older than 24 hours and creates a new one if needed,
   ensuring all users have recent backups regardless of usage patterns.

3. Backfill failure notification - switch now returns SwitchResult with
   warnings instead of silently ignoring backfill errors, so users are
   informed when their manual config changes may not have been saved.

4. Backup management UI - new BackupListSection in Settings > Data
   Management showing all backup snapshots with restore capability,
   including a confirmation dialog and automatic safety backup before
   restore.

src-tauri/src/services/mod.rs

@@ -18,7 +18,7 @@ pub use config::ConfigService;
 pub use mcp::McpService;
 pub use omo::OmoService;
 pub use prompt::PromptService;
-pub use provider::{ProviderService, ProviderSortUpdate};
+pub use provider::{ProviderService, ProviderSortUpdate, SwitchResult};
 pub use proxy::ProxyService;
 #[allow(unused_imports)]
 pub use skill::{DiscoverableSkill, Skill, SkillRepo, SkillService};