Include the currently-playing episode in the Next Up queue (issue #196)

[mapitman]

Summary

The currently-playing episode was fully removed from the queue table when it started playing (issue #171) and was only ever visible via the mini/expanded player -- Next Up itself had no idea it existed. Per discussion on the issue, it's now a real queue entry like any other, always kept at the front, and shown in the list clearly marked "Now Playing" instead of hidden.

• QueueRepository: moveToFront() replaces the old remove-on-play; peekFront() replaces the old pop-and-remove popNext() (advancing to an episode shouldn't dequeue it now that it's meant to stay queued while playing); moveToEnd() gets a failed episode out of the front slot without discarding it outright.
• PlaybackController.play()/restoreLastPlayingItem() move the episode to the queue's front instead of removing it. requeuePreviousEpisode() (re-add if unfinished) becomes dropFromQueueIfFinished() (remove if it turns out to have finished) -- the previous episode never actually left the queue this time.
• PlaybackService: the STATE_ENDED handler drops the finished episode's row itself now, deferred until after playNextQueued() so issue Substantial silence gap between Next Up episodes #82's silence-gap fix isn't undone. onPlayerError moves the failed episode to the back instead (it never actually finished, so it's still worth surfacing to retry). Both pass an excluded itemId into playNextQueued() so it doesn't just re-select the very episode it's meant to be skipping past -- needed because a lone queued episode moved to the back is still the only (and therefore front) entry. startPreloadingQueueHead() now skips the currently-playing entry when picking what to preload next, for the same reason.
• No UI filtering needed for the queue list: ReorderableQueueList already fully supported an isCurrentlyPlaying row (highlight, disabled long-press-to-play, etc.) from when that state was only ever transient; it just needed the explicit "Now Playing" label (small play icon + text, swapped in for the date).

Test plan

• ./gradlew assembleDebug testDebugUnitTest lintDebug -- all green
• Updated/added unit tests: QueueRepositoryTest (peekFront/moveToFront/moveToEnd), PlaybackControllerTest (play moves to front instead of removing), QueueViewModelTest (playNow moves to front instead of shrinking the queue)
• Manual on-device verification (emulator, API 31): played a downloaded episode, opened Next Up -- it appears at the top, highlighted, with a play icon and "Now Playing" label; the rest of the queue is unaffected below it. No crash.
• No coverage for PlaybackService's advance-on-error/end logic itself -- same pre-existing constraint as issues Podcasts stop playing intermittently  #201/Chapter navigation is no longer available  #202's PRs (no MediaSessionService/ExoPlayer test harness in this repo).

Closes #196