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/components/settings/SettingsPage.tsx

@@ -33,6 +33,7 @@ import { SkillSyncMethodSettings } from "@/components/settings/SkillSyncMethodSe
 import { TerminalSettings } from "@/components/settings/TerminalSettings";
 import { DirectorySettings } from "@/components/settings/DirectorySettings";
 import { ImportExportSection } from "@/components/settings/ImportExportSection";
+import { BackupListSection } from "@/components/settings/BackupListSection";
 import { WebdavSyncSection } from "@/components/settings/WebdavSyncSection";
 import { AboutSection } from "@/components/settings/AboutSection";
 import { ProxyTabContent } from "@/components/settings/ProxyTabContent";
@@ -324,6 +325,7 @@ export function SettingsPage({
                             onExport={exportConfig}
                             onClear={clearSelection}
                           />
+                          <BackupListSection />
                         </AccordionContent>
                       </AccordionItem>