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/lib/api/providers.ts

@@ -17,6 +17,10 @@ export interface ProviderSwitchEvent {
   providerId: string;
 }
 
+export interface SwitchResult {
+  warnings: string[];
+}
+
 export const providersApi = {
   async getAll(appId: AppId): Promise<Record<string, Provider>> {
     return await invoke("get_providers", { app: appId });
@@ -46,7 +50,7 @@ export const providersApi = {
     return await invoke("remove_provider_from_live_config", { id, app: appId });
   },
 
-  async switch(id: string, appId: AppId): Promise<boolean> {
+  async switch(id: string, appId: AppId): Promise<SwitchResult> {
     return await invoke("switch_provider", { id, app: appId });
   },