feat(slint): Last.fm and ListenBrainz scrobblers wired to player and settings

Re-implements the parked scrobblers-wip stash intent on the current tree
(the stash no longer applied; it remains untouched in the stash list) with
three corrections: the Last.fm offline queue uses the SHARED per-user
offline_settings.db scrobble_queue (byte-identical SQL to Tauri - queues
flush from either frontend) instead of a private table; the ListenBrainz
cache opens Tauri's per-user path so LB creds and listen_queue are truly
shared (a Tauri LB sign-in pre-seeds Slint); and the queue flush is driven
by the offline engine's online edge (plus initial flush), not login-only.

New frontend-agnostic qbz_app::settings::scrobblers store (master enable,
Last.fm session/username, LB token/username; tests) and the shared
scrobble-queue API on OfflineModeStore. Player wiring hooks the de-duped
track-change edge inside the QConnect remote gate: now-playing fires on
change (skipped offline), the scrobble arms at min(50%, 240s) with a
generation counter cancelling stale timers, all sources scrobble (Tauri
parity incl. the #360 version-enriched title). Offline or call failure
queues; Last.fm flush honors the 50-batch and 14-day cutoff.

Settings gains an Integrations section (Plex-section idiom): Last.fm
browser-auth connect/finish/disconnect, ListenBrainz token field +
validate, per-service enable toggles. Known gap documented: Last.fm creds
cannot be shared with Tauri (webview localStorage); LB creds and both
queues are shared.

cc

crates/qbz-slint/ui/state.slint

@@ -854,6 +854,52 @@ export global PlexAuthActions {
     callback load();                                 // panel init: seed from store + warm cache
 }
 
+// ============================ Scrobblers ==================================
+// Settings > Integrations — source-agnostic Last.fm + ListenBrainz scrobbling.
+// State mirrors `scrobbler_settings.db`; actions delegate to the Rust
+// `scrobble` controller (auth flows + the now-playing/scrobble fire live
+// there). ADR-008: no pills.
+
+export global ScrobbleState {
+    // --- Master section toggles -------------------------------------------
+    in-out property <bool> enabled: false;            // master toggle (default OFF)
+    in-out property <bool> ui-collapsed: false;       // collapse chevron
+
+    // --- Last.fm ----------------------------------------------------------
+    in-out property <bool> lastfm-enabled: false;     // per-service scrobble flag
+    in property <bool> lastfm-authed: false;          // session key present
+    in property <string> lastfm-username: "";         // "Signed in as ..."
+    in property <string> lastfm-auth-url: "";         // non-empty after connect
+    in property <bool> lastfm-busy: false;            // get_token / get_session in flight
+
+    // --- ListenBrainz -----------------------------------------------------
+    in-out property <bool> listenbrainz-enabled: false;
+    in property <bool> listenbrainz-authed: false;
+    in property <string> listenbrainz-username: "";
+    in-out property <string> listenbrainz-token-input: ""; // token field buffer
+    in property <bool> listenbrainz-busy: false;      // set_token in flight
+
+    // --- Shared status line (0 none, 1 info, 2 ok, 3 error) ---------------
+    in property <string> status-text: "";
+    in property <int> status-kind: 0;
+}
+
+export global ScrobbleActions {
+    callback load();                                  // panel init: seed from store
+    callback enable-toggle(bool);
+    callback collapse-toggle(bool);
+    // Last.fm.
+    callback lastfm-enable-toggle(bool);
+    callback lastfm-connect();                        // get_token + open browser
+    callback lastfm-open-auth-url();                  // re-open authorize URL
+    callback lastfm-confirm();                        // get_session after approve
+    callback lastfm-disconnect();
+    // ListenBrainz.
+    callback listenbrainz-enable-toggle(bool);
+    callback listenbrainz-set-token(string);
+    callback listenbrainz-disconnect();
+}
+
 // ============================ Tag editor ==================================
 // Local-album metadata editor (Settings-independent modal launched from the
 // local album detail). Sidecar (default) + opt-in direct file-write. Remote