feat(local-library): add a Plex sync button to refresh media without restarting

Fetching new Plex music required restarting QBZ. Add a sync button in the Local Library header (gated on plex-available, with a busy spinner) that triggers the existing Plex re-sync (new plex_auth::sync_now over the persisted selected libraries) and refreshes the view. Strings in all 8 locales. Resolves #573.

cc

crates/qbz-ui/ui/state.slint

@@ -1056,6 +1056,11 @@ export global LocalLibraryState {
     in-out property <string> active-tab: "albums";
     in property <bool> loading: false;
     in property <string> load-error: "";
+    // Plex manual sync (#573): `plex-available` gates the header Sync button
+    // (Plex enabled + authenticated — seeded at startup from the store, kept
+    // fresh by plex_auth's gate refresh); `plex-syncing` drives its spinner.
+    in property <bool> plex-available: false;
+    in property <bool> plex-syncing: false;
     // Header stats / per-tab badge counts.
     in property <int> album-count: 0;
     in property <int> artist-count: 0;
@@ -1197,6 +1202,10 @@ export global LocalLibraryActions {
     callback select-tab(string /* "albums"|"artists"|"folders"|"tracks" */);
     // Open Settings > Local Library (the gear button — management lives there).
     callback open-settings();
+    // Manual Plex re-sync (#573) — the header Sync button, visible only when
+    // Plex is configured. Re-fetches sections + tracks, then the Rust side
+    // reloads the browse models in place.
+    callback sync-plex();
 
     // --- Albums tab ---
     // Re-query page 1 with the current search text (server-side LIKE).