feat(slint): play-fetching spinner (now-playing bar + track row + album button)

A play can take seconds before audio starts (esp. Plex, whose resolve downloads
the whole file) with no feedback today. Add a source-agnostic loading indicator:

- A loading flag keyed to PENDING_PLAY_ID, raised at the top of play_audible
  (before the Plex/Qobuz/local resolve — single chokepoint, covers auto-advance)
  and cleared by the poll loop when the requested track's audio actually starts
  (position > 0). Cleared on every play-error early-return and force-cleared on
  queue-finished / peer-controller / ephemeral-stop so it never sticks.
- Watchdog: force-clear after 45s if the engine accepted a play that silently
  never advances (undecodable-but-valid file) — otherwise the spinner is eternal.
- Spinner on three surfaces driven by NowPlayingState.loading: now-playing bar
  (cover overlay + play/pause button), the active track row's play cell, and the
  album big play button (gated to the album actually being launched).

Audio backend / playback routing untouched. The spinner only communicates the
wait; the underlying Plex fetch latency is fixed separately (progressive streaming).

cc

crates/qbz-slint/ui/state.slint

@@ -2106,6 +2106,12 @@ export global NowPlayingState {
     // it has no DB row, so favorite / add-to-playlist / track-info are gated off.
     in property <bool> is-ephemeral: false;
     in property <bool> playing: false;
+    // Fetch/resolve in flight for the track being started — no audio yet.
+    // Set the instant a play is requested (before the Plex/Qobuz/local
+    // resolve) and cleared once the poll loop sees the audio advancing.
+    // Drives the fetch spinner on the bar, the active track row, and the
+    // album play button.
+    in property <bool> loading: false;
     in property <int> position-secs: 0;
     in property <int> duration-secs: 0;
     in property <float> progress: 0.0;   // 0..1