feat(backup): add independent backup panel, configurable policy, and rename support

Extract backup & restore into a standalone AccordionItem in Advanced settings.
Add configurable auto-backup interval (disabled/6h/12h/24h/48h/7d) and retention
count (3-50) via settings. Add per-backup rename with inline editing UI.

src/lib/api/settings.ts

@@ -230,4 +230,8 @@ export const backupsApi = {
   async restoreDbBackup(filename: string): Promise<string> {
     return await invoke("restore_db_backup", { filename });
   },
+
+  async renameDbBackup(oldFilename: string, newName: string): Promise<string> {
+    return await invoke("rename_db_backup", { oldFilename, newName });
+  },
 };