Merge the deduplication and dead-code removal pass

Consolidates duplicated logic behind shared implementations — paginated
grid tabs, focus chrome, cached remote stores, sheet selection columns,
the server artifact store and a test fixture layer — and removes code
that had become unreachable. Net reduction of about 5,500 lines with no
behaviour change.

Where a fix had landed separately in code that moved into a shared
helper, the fix was re-applied inside the helper rather than left behind
in the copy that went away.

lib/widgets/video_controls/parts/track_controls.dart

@@ -144,7 +144,6 @@ extension _PlexVideoControlsTrackMethods on _PlexVideoControlsState {
       audioSyncOffset: _audioSyncOffset,
       subtitleSyncOffset: _subtitleSyncOffset,
       isRotationLocked: _isRotationLocked,
-      isScreenLocked: _isScreenLocked,
       isFullscreen: _isFullscreen,
       isAlwaysOnTop: _isAlwaysOnTop,
       onTogglePIPMode: (_isPipSupported && !PlatformDetector.isTV()) ? widget.onTogglePIPMode : null,