Add setting to hide already-played tracks in play queue

[belzebub40k]

Summary

• Add a setting to hide already-played tracks from the play queue, so the current track stays at the top and the queue shows what is coming up
• The toggle is available both in Settings > General ("Hide played tracks from queue") and directly in the play queue popup ("Hide played tracks")
• All queue views (Now Playing page, sidebar, popup) follow the setting and update immediately when it is toggled
• Displayed track numbers keep the track's position in the full queue instead of restarting from 1
• Clicking, removing and drag-reordering a row in the filtered view act on the intended track

Changes

• Added HidePlayedQueueTracks config option
• Added the checkbox to Settings > General and to the play queue popup, kept in sync with each other
• PlayQueueList owns the play queue and the playing index: SetQueue, SetNowPlayingIndex and SetHidePlayed apply the filter internally, and the indexes reported to OnPlayItemAt, OnRemoveFromQueue and OnReorderItems are translated back into full-queue indexes. Queue() exposes the unfiltered queue to the handlers that receive those indexes
• A track change only rebuilds the displayed rows when it changes which rows are hidden, so an ordinary track transition doesn't discard the user's selection
• The views are notified of a setting change through Controller.HidePlayedTracksChangedFunc, injected by MainWindow alongside its other view-level callbacks
• Fixed track numbers resetting to 1 when tracks are hidden
• Fixed scroll position on initial load, page reload and widget-pool recycling

Tests

ui/widgets/playqueuelist_test.go covers the display/queue index translation, the displayed track numbers, the reorder contract, selection preservation across a track change, and the cases where nothing should be hidden (nothing playing, first track, playing index past the end of the queue, empty queue).

Manual testing

• Toggle the setting in Settings > General and in the queue popup, and verify all queue views update immediately and the two checkboxes stay in sync
• With played tracks hidden: click a row to play it, remove a selection, and drag to reorder — verify the intended tracks are affected and that no tracks disappear from the queue
• Verify displayed track numbers continue from the current track
• Navigate between pages and reload, verifying scroll position behaves correctly

[belzebub40k]

Updated to retain track number in queue.

[belzebub40k]

Added checkbox to quickly toggle the setting in the play queue popup.

[belzebub40k]

Rebased onto current main (the module rename and the now-padded settings dialog were the only conflicts) and went back over the whole diff. A few real bugs turned up:

• "Remove from queue" removed the wrong tracks. It passed indexes of the filtered list to RemoveTracksFromQueue, which reads them as indexes into the full queue, so it deleted already-hidden tracks instead of the selected ones.
• Drag-reordering could crash the app. OnReorderItems reports full-queue indexes but the handler paired them with the displayed items: index out of range [9] with length 5. When it didn't panic it moved the wrong track and rebuilt the queue from the visible items alone, silently dropping every already-played track.
• The queue list was rebuilt on every track change even with the setting off, which recreates the track models and so cleared the user's selection — that one affected people not using the feature.
• The Now Playing queue jumped to the top on any queue change, because the scroll reset sat in Reload(), which is also the queue-change handler. It now only runs on first load, which is what it was for.
• The two new strings were missing from en.json, and the popup checkbox didn't persist the setting.

Cleanups:

• The filter now lives in PlayQueueList rather than being re-applied by each of the three queue views, which all had to call SetPlayIndexOffset and SetItems together and in the right order or the displayed rows and the engine-facing indexes would disagree. That coupling is what caused both index bugs. FilterQueueForHidePlayed and SetPlayIndexOffset are gone.
• Dropped PlaybackManager.TriggerQueueChangeCallback. Having the backend fabricate a queue-change event for a UI-only setting was the wrong shape; the views are notified from MainWindow now, like the Controller's other view-level callbacks.

Added ui/widgets/playqueuelist_test.go (8 tests) covering the display↔queue index translation, the displayed track numbers, the reorder contract, and the cases where nothing should be hidden. I checked each test fails when the corresponding bug is put back.

The settings checkbox now reads "Hide played tracks from queue"; the popup keeps the shorter "Hide played tracks".

Pushed on top of the existing commits, and I've updated the description above to match.