refactor: extract shared mixins and helpers, drop dead abstractions

Introduces shared seams for paginated views, D-pad reorder, media control
routing, async singletons and the device method channel, then points the
open-coded copies at them.

Also removes unused models and duplicated provider/server plumbing, folds
the twice-implemented artifact store in the server, and factors the
repeated Flutter toolchain prologue in CI into a composite action.

lib/media/library_query.dart

@@ -1,6 +1,7 @@
 // ignore_for_file: invalid_annotation_target
 import 'package:freezed_annotation/freezed_annotation.dart';
 
+import '../utils/media_server_http_client.dart' show AbortController;
 import 'media_kind.dart';
 
 part 'library_query.freezed.dart';
@@ -84,3 +85,33 @@ int fallbackPageTotal({required int offset, required int itemCount, int? request
   final fullPage = requestedSize != null && requestedSize > 0 && itemCount >= requestedSize;
   return offset + itemCount + (fullPage ? 1 : 0);
 }
+
+/// Walk every page of a paginated endpoint and concatenate the results.
+///
+/// [fetchPage] receives a zero-based offset and [pageSize] and is called until
+/// a page comes back empty, the accumulated count reaches the page's
+/// [LibraryPage.totalCount], or — when [stopOnShortPage] is set — a page comes
+/// back shorter than [pageSize]. The short-page break is for backends whose
+/// total is unreliable; leave it off when the total is authoritative.
+///
+/// [abort] is checked before and after every request. Errors propagate.
+Future<List<T>> drainPages<T>(
+  Future<LibraryPage<T>> Function(int start, int size) fetchPage, {
+  required int pageSize,
+  AbortController? abort,
+  bool stopOnShortPage = false,
+}) async {
+  final all = <T>[];
+  var start = 0;
+  while (true) {
+    abort?.throwIfAborted();
+    final page = await fetchPage(start, pageSize);
+    abort?.throwIfAborted();
+    if (page.items.isEmpty) break;
+    all.addAll(page.items);
+    start += page.items.length;
+    if (start >= page.totalCount) break;
+    if (stopOnShortPage && page.items.length < pageSize) break;
+  }
+  return all;
+}

lib/media/server_capabilities.dart

@@ -212,55 +212,35 @@ class ServerCapabilities {
     audioTranscoding: true,
   );
 
-  ServerCapabilities copyWith({
-    bool? serverSidePlayQueue,
-    bool? serverSidePlaylists,
-    bool? liveTv,
-    bool? liveTvDvr,
-    bool? subtitleSearch,
-    bool? videoTranscoding,
-    bool? serverSideSync,
-    bool? richHubs,
-    bool? numericUserRating,
-    bool? userFavorites,
-    bool? continueWatchingRemoval,
-    bool? externalSubtitleSearch,
-    bool? trackPreferencePersistence,
-    bool? endpointFailover,
-    bool? offlineWatchQueue,
-    bool? discordRpc,
-    bool? richMetadataEdit,
-    AlphaBarMode? alphaBar,
-    bool? scrubThumbnails,
-    bool? folderGrouping,
-    bool? lyrics,
-    bool? instantMix,
-    bool? audioTranscoding,
-  }) {
+  /// Every flag here is fixed per backend *kind* except [videoTranscoding],
+  /// which Plex probes per server (`PlexClient.capabilities`) — so that is the
+  /// only override this type needs. Widen the parameter list if another flag
+  /// ever becomes a runtime probe.
+  ServerCapabilities copyWith({bool? videoTranscoding}) {
     return ServerCapabilities(
-      serverSidePlayQueue: serverSidePlayQueue ?? this.serverSidePlayQueue,
-      serverSidePlaylists: serverSidePlaylists ?? this.serverSidePlaylists,
-      liveTv: liveTv ?? this.liveTv,
-      liveTvDvr: liveTvDvr ?? this.liveTvDvr,
-      subtitleSearch: subtitleSearch ?? this.subtitleSearch,
+      serverSidePlayQueue: serverSidePlayQueue,
+      serverSidePlaylists: serverSidePlaylists,
+      liveTv: liveTv,
+      liveTvDvr: liveTvDvr,
+      subtitleSearch: subtitleSearch,
       videoTranscoding: videoTranscoding ?? this.videoTranscoding,
-      serverSideSync: serverSideSync ?? this.serverSideSync,
-      richHubs: richHubs ?? this.richHubs,
-      numericUserRating: numericUserRating ?? this.numericUserRating,
-      userFavorites: userFavorites ?? this.userFavorites,
-      continueWatchingRemoval: continueWatchingRemoval ?? this.continueWatchingRemoval,
-      externalSubtitleSearch: externalSubtitleSearch ?? this.externalSubtitleSearch,
-      trackPreferencePersistence: trackPreferencePersistence ?? this.trackPreferencePersistence,
-      endpointFailover: endpointFailover ?? this.endpointFailover,
-      offlineWatchQueue: offlineWatchQueue ?? this.offlineWatchQueue,
-      discordRpc: discordRpc ?? this.discordRpc,
-      richMetadataEdit: richMetadataEdit ?? this.richMetadataEdit,
-      alphaBar: alphaBar ?? this.alphaBar,
-      scrubThumbnails: scrubThumbnails ?? this.scrubThumbnails,
-      folderGrouping: folderGrouping ?? this.folderGrouping,
-      lyrics: lyrics ?? this.lyrics,
-      instantMix: instantMix ?? this.instantMix,
-      audioTranscoding: audioTranscoding ?? this.audioTranscoding,
+      serverSideSync: serverSideSync,
+      richHubs: richHubs,
+      numericUserRating: numericUserRating,
+      userFavorites: userFavorites,
+      continueWatchingRemoval: continueWatchingRemoval,
+      externalSubtitleSearch: externalSubtitleSearch,
+      trackPreferencePersistence: trackPreferencePersistence,
+      endpointFailover: endpointFailover,
+      offlineWatchQueue: offlineWatchQueue,
+      discordRpc: discordRpc,
+      richMetadataEdit: richMetadataEdit,
+      alphaBar: alphaBar,
+      scrubThumbnails: scrubThumbnails,
+      folderGrouping: folderGrouping,
+      lyrics: lyrics,
+      instantMix: instantMix,
+      audioTranscoding: audioTranscoding,
     );
   }
 }