Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@lib/providers/player_provider.dart`:
- Around line 683-687: The current isRemotePlayback getter only checks
connection flags (_upnpService.isConnected || _castService.isConnected) which
misclassifies local playback as remote; introduce an explicit boolean state
(e.g., _isRenderingRemotely) used by isRemotePlayback instead of raw connection
state, set _isRenderingRemotely = true when you actually start playback on a
remote renderer and set it = false when you drive the local _audioPlayer (see
playRadioStation and any code paths that call
_audioPlayer.play/_audioPlayer.stop), and also clear _isRenderingRemotely on
remote stop/disconnect events and on errors from _castService/_upnpService so
the audio-focus/noisy handlers and volume routing reflect the active output.
- Around line 1547-1548: The dispose/shutdown sequence currently closes
_positionController while the listeners on playerStateStream, positionStream,
and durationStream remain active; store those stream subscriptions (e.g., as
StreamSubscription fields or in a List<StreamSubscription>) when you create them
and call cancel() on each subscription before calling
_positionController.close() (and after/around _discordRpcService.shutdown()).
Update the class to keep references to the subscriptions (names like
_playerStateSub, _positionSub, _durationSub or a single _playerSubscriptions)
and ensure all are cancelled to prevent late just_audio ticks from calling
_positionController.add after disposal.

In `@lib/screens/now_playing_screen.dart`:
- Around line 2628-2637: The drag handler (_applyVolume) must keep the
optimistic UI update of _dragValue/_systemVolume but serialize remote commits to
avoid out-of-order provider.setVolume() completions; add instance fields like
_pendingRemoteVolume (double?), and _remoteCommitInProgress (bool) and when
isRemote is true, assign _pendingRemoteVolume = newVolume and kick off a single
async commit loop (if !_remoteCommitInProgress) that repeatedly reads the latest
_pendingRemoteVolume, awaits provider.setVolume(value) and then checks whether a
newer value arrived before finishing; do the same serialization change for the
other remote-write site (around the 2679-2696 block) so only the latest value is
actually awaited/sent and intermediate ticks are coalesced.

In `@lib/services/upnp_service.dart`:
- Around line 298-312: The _consecutivePollErrors counter is mutated without
notifying observers; update the code in the method that handles polling (the
block where state == null) to call notifyListeners() after incrementing
_consecutivePollErrors, and likewise call notifyListeners() after resetting
_consecutivePollErrors to 0 (the reset around lines 339-340); reference the
private field _consecutivePollErrors and the public getter consecutivePollErrors
so the changes notify any listeners watching that state.