fix(queue): clicking a track respects shuffle order (#327)

The frontend was resolving queue-panel clicks via
`actualIndex = current_index + trackIndex`, mixing the unshuffled
canonical position with the shuffle-aware display position. On shuffle
mode, the resulting index pointed at a different track than the one
the user clicked, which is exactly what issue #327 reports.

Solution:
- New `QueueManager::play_upcoming_at(upcoming_index)` resolves the
  click position through `shuffle_order` + `shuffle_position` when
  shuffle is on, and falls back to `current_index + upcoming_index + 1`
  otherwise.
- Exposed through `QbzCore::play_upcoming_at` (emits QueueUpdated) and
  `CoreBridge::play_upcoming_at`.
- New `v2_play_queue_upcoming_at` tauri command.
- `QueuePanel.onPlayTrack` now carries the `originalIndex` so the
  caller knows the exact upcoming-list position the user clicked.
- `handleQueueTrackPlay` calls the new command directly — no more
  client-side index arithmetic.

Also deletes the two dead `From<crate::queue::QueueTrack>` impls in
commands_v2/queue.rs (canonical queue lives in qbz-player now), per
the zero-legacy V2 policy.

Tests: two new unit tests cover the linear and shuffle-aware paths.
Session persistence is untouched — the new method only consults
existing state.

cc

crates/qbz-core/src/core.rs

@@ -327,6 +327,19 @@ impl<A: FrontendAdapter + Send + Sync + 'static> QbzCore<A> {
         track
     }
 
+    /// Jump to a track by its position in the upcoming list (as shown in the
+    /// Queue sidebar). Shuffle-aware: resolves through `shuffle_order` when
+    /// shuffle is active.
+    pub async fn play_upcoming_at(&self, upcoming_index: usize) -> Option<QueueTrack> {
+        let queue = self.queue.write().await;
+        let track = queue.play_upcoming_at(upcoming_index);
+        self.emit(CoreEvent::QueueUpdated {
+            state: queue.get_state(),
+        })
+        .await;
+        track
+    }
+
     /// Advance to next track in queue
     pub async fn next_track(&self) -> Option<QueueTrack> {
         let queue = self.queue.write().await;