Files backend: release-name parser, video-only scanner, folder store, mpv thumbnails

lib/src/files/ for the 1.0 Files source (UI comes with the library
round; ARCHITECTURE gets its section then):
- parseReleaseName: series/season/episode/group/year/extras from release
  names (fansub, scene, batch, NCOP/OVA markers); seriesKey for grouping.
- scanFolder (isolate): videos only, samples/trailers and other files
  counted as hidden, sidecar subtitles paired by name (+ language/flag
  suffixes), 20 000-file cap; groupBySeries -> series + loose videos.
- FolderStore: watched folders and recent files in Hive, as signals.
- thumbnailFor: a 480 px JPEG at 25% via a hidden mpv player
  (screenshot-to-file, vo=null), cached by path+size+mtime, 2 at a time,
  cancellable, never throws. No ffmpeg needed (Windows ships only libmpv).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/test/files_store_test.dart

@@ -0,0 +1,48 @@
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:hive_ce/hive.dart';
+import 'package:watch_together/src/files/folder_store.dart';
+
+void main() {
+  test('folders and recent files persist across a reload', () async {
+    final dir = Directory.systemTemp.createTempSync('files_store_');
+    addTearDown(() => dir.deleteSync(recursive: true));
+    Hive.init(dir.path);
+
+    final a = FolderStore();
+    await a.ensureLoaded();
+    expect(a.folders.value, isEmpty);
+    expect(a.recent.value, isEmpty);
+    expect(a.addFolder('/media/anime'), isTrue);
+    expect(a.addFolder('/media/movies'), isTrue);
+    expect(a.addFolder('/media/anime'), isFalse);
+    a.removeFolder('/media/movies');
+    a.addFolder('/media/films');
+    for (var i = 0; i < 35; i++) {
+      a.addRecent('/v/$i.mkv');
+    }
+    a.addRecent('/v/10.mkv'); // back to the top, not duplicated
+    a.removeRecent('/v/33.mkv');
+    expect(a.recent.value.first, '/v/10.mkv');
+    expect(a.recent.value, hasLength(maxRecentFiles - 1));
+    expect(a.recent.value.where((p) => p == '/v/10.mkv'), hasLength(1));
+    await Future.delayed(const Duration(milliseconds: 100));
+    await Hive.close();
+
+    final b = FolderStore();
+    await b.ensureLoaded();
+    expect(b.folders.value, ['/media/anime', '/media/films']);
+    expect(b.recent.value, a.recent.value);
+    expect(b.recent.value.take(3), ['/v/10.mkv', '/v/34.mkv', '/v/32.mkv']);
+    b.clearRecent();
+    await Future.delayed(const Duration(milliseconds: 100));
+    await Hive.close();
+
+    final c = FolderStore();
+    await c.ensureLoaded();
+    expect(c.recent.value, isEmpty);
+    expect(c.folders.value, hasLength(2));
+    await Hive.close();
+  });
+}