fix(explore): list every library copy of a title, not one per server

`MediaServerClient.findByExternalIds` returned `MediaItem?`, so the Explore
"In these libraries" chooser could never show more than one copy per server.
A movie held by both a 4K library and an HD library on one Plex server
therefore resolved to whichever copy came back first, with no way to reach
the other.

Return every id-verified match instead. `/library/all` is already
server-wide and each `Metadata` entry carries its own `librarySectionID`,
so both copies come back labelled with no extra request; Plex was simply
taking `Metadata[0]` and the title ladder was returning on its first hit.
An exact-guid hit no longer short-circuits the title search either — a
library still on a legacy agent has a different primary guid and is
invisible to the `guid=` filter.

Copies are deduped by global key and ordered best-first, and each row now
states its resolution, since library names need not mention it.

Resolution passes merge rather than replace: the cross-server fan-out logs
and skips per-server failures, so a later pass can come back short a server
that answered an earlier one, and a failed pass no longer claims the title
left the library. Duplicate keys fold field by field, because Jellyfin's
library stamp is a best-effort ancestors lookup that returns the item bare
when it fails and an unstamped row is indistinguishable from its sibling.
Focus nodes are keyed by copy and reclaimed after a merge re-sorts the
rows, so a dpad user is not thrown to a different copy.

close #1754

test/media/media_item_merge_test.dart

@@ -4,6 +4,7 @@ import 'package:plezy/media/media_backend.dart';
 import 'package:plezy/media/media_item.dart';
 import 'package:plezy/media/media_item_merge.dart';
 import 'package:plezy/media/media_kind.dart';
+import 'package:plezy/media/media_version.dart';
 import '../test_helpers/media_items.dart';
 
 void main() {
@@ -47,4 +48,69 @@ void main() {
     expect(merged.libraryId, 'old-lib');
     expect(merged.libraryTitle, 'Old');
   });
+
+  MediaItem copy({
+    required String id,
+    String? libraryTitle,
+    String? videoResolution,
+    List<MediaVersion>? mediaVersions,
+  }) => testMediaItem(
+    id: id,
+    serverId: 'server-1',
+    serverName: 'Living Room',
+    libraryId: libraryTitle == null ? null : '$id-lib',
+    libraryTitle: libraryTitle,
+    mediaVersions:
+        mediaVersions ??
+        (videoResolution == null ? null : [MediaVersion(id: '$id-v', videoResolution: videoResolution)]),
+  );
+
+  test('orders library copies by resolution, best first', () {
+    final merged = mergeLibraryCopies(const [], [
+      copy(id: 'hd', libraryTitle: 'Movies', videoResolution: '1080'),
+      copy(id: 'uhd', libraryTitle: '4K Movies', videoResolution: '4k'),
+    ]);
+
+    expect(merged.map((item) => item.id), ['uhd', 'hd']);
+  });
+
+  test('keeps copies a later pass did not return', () {
+    final merged = mergeLibraryCopies([copy(id: 'hd', libraryTitle: 'Movies')], const []);
+
+    expect(merged.map((item) => item.id), ['hd']);
+  });
+
+  test('an unstamped re-resolve does not strip a copy of its library', () {
+    // Jellyfin's library stamp is a best-effort ancestors lookup that returns
+    // the item unstamped when it fails. Losing the title would make this copy
+    // indistinguishable from its sibling in the server's other library.
+    final merged = mergeLibraryCopies(
+      [copy(id: 'hd', libraryTitle: 'Movies', videoResolution: '1080')],
+      [copy(id: 'hd')],
+    );
+
+    expect(merged.single.libraryId, 'hd-lib');
+    expect(merged.single.libraryTitle, 'Movies');
+    expect(merged.single.serverName, 'Living Room');
+    expect(merged.single.mediaVersions?.single.videoResolution, '1080');
+  });
+
+  test('a re-resolve carrying fresher library context wins', () {
+    final merged = mergeLibraryCopies(
+      [copy(id: 'hd', libraryTitle: 'Movies', videoResolution: '1080')],
+      [copy(id: 'hd', libraryTitle: 'Renamed Movies', videoResolution: '4k')],
+    );
+
+    expect(merged.single.libraryTitle, 'Renamed Movies');
+    expect(merged.single.mediaVersions?.single.videoResolution, '4k');
+  });
+
+  test('an empty version list is treated as absent, not as a downgrade', () {
+    final merged = mergeLibraryCopies(
+      [copy(id: 'hd', libraryTitle: 'Movies', videoResolution: '1080')],
+      [copy(id: 'hd', libraryTitle: 'Movies', mediaVersions: const [])],
+    );
+
+    expect(merged.single.mediaVersions?.single.videoResolution, '1080');
+  });
 }