Persist last-opened library file across restarts; fix path-switch bug

- Remember which library.db was last chosen (via a small config.json in
  the app-data dir) so relaunching the app reopens the same file instead
  of always defaulting to library.db.
- Fix set_library_path returning a hard error (and thus never updating
  the topbar) whenever the reference catalog fails to reopen against the
  new path — e.g. a stale reference.db schema. Picking a library file
  and successfully reattaching the reference db are separate concerns;
  the chosen path now always persists and is reported back, with the
  catalog reopened best-effort.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src-tauri/src/commands.rs

@@ -40,8 +40,12 @@ pub fn get_status(db: Db) -> Res<Status> {
 #[tauri::command]
 pub fn set_library_path(db: Db, path: String) -> Res<Status> {
     let mut s = db.lock().unwrap();
-    s.library_path = PathBuf::from(path);
-    s.reopen()?;
+    s.set_library_path(PathBuf::from(path));
+    // Best-effort reopen: the chosen path must stick (and be reported back)
+    // even if the reference db can't currently be attached (e.g. stale
+    // schema pending an "Update card data" run) — that's a separate concern
+    // from which library file is open.
+    let _ = s.reopen();
     Ok(status_of(&s))
 }