Shaders: Anime4K presets in-session and in Settings

Vendored Anime4K v4.0.1 GLSL (MIT), extracted to app support once per
version. Presets A/B/C/A+A/B+B/C+A in HQ and Fast per Anime4K's mpv
templates, plus custom files. Shader button in the video controls
(works in fullscreen), Ctrl+1..6 / Ctrl+0, live apply with read-back,
failures fall back to Off with a toast. Render mode detected from
mpv's GL log; warns when on the software path. Docs include the
Jellyfin client section.

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

apps/desktop/test/shaders_mpv_test.dart

@@ -0,0 +1,75 @@
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:media_kit/media_kit.dart';
+import 'package:watch_together/src/playback/playback_controller.dart';
+import 'package:watch_together/src/shaders/shader_assets.dart';
+import 'package:watch_together/src/shaders/shader_presets.dart';
+import 'package:watch_together/src/state/settings.dart';
+
+final _tmp = Platform.environment['WT_TEST_TMP'] ?? Directory.systemTemp.path;
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  // Headless mpv (no video output) still takes and reports glsl-shaders.
+  test('real mpv: presets set glsl-shaders live, failures fall back to off',
+      timeout: const Timeout(Duration(minutes: 1)), () async {
+    MediaKit.ensureInitialized();
+    final support = Directory('$_tmp/shaders_mpv_support');
+    if (support.existsSync()) support.deleteSync(recursive: true);
+    support.createSync(recursive: true);
+    shaderSupportDir = () async => support;
+    final s = AppSettings.instance;
+    final p = PlaybackController();
+    try {
+      // The saved choice goes on at player creation.
+      s.setShaderPreset(ShaderPreset.a);
+      await p.init();
+      final dynamic mpv = p.player.platform;
+      final modeA = joinShaderPaths(
+          await shaderPathsFor(ShaderPreset.a, ShaderQuality.hq, const []));
+      expect(await mpv.getProperty('glsl-shaders'), modeA);
+      expect(p.shaderNotice.value, isNull);
+
+      // Live change, announced.
+      s.setShaderPreset(ShaderPreset.ca);
+      s.setShaderQuality(ShaderQuality.fast);
+      await p.applyShaders(announce: true);
+      final caFast = await shaderPathsFor(
+          ShaderPreset.ca, ShaderQuality.fast, const []);
+      expect(caFast.first, startsWith(support.path));
+      expect(await mpv.getProperty('glsl-shaders'), joinShaderPaths(caFast));
+      expect(p.shaderNotice.value?.text, 'Shaders: Mode C+A (Fast)');
+
+      // A missing custom file: toast, off, nothing set.
+      final good = File('${support.path}/mine.glsl')..writeAsStringSync('//');
+      s.setCustomShaders([good.path, '${support.path}/gone.glsl']);
+      s.setShaderPreset(ShaderPreset.custom);
+      await p.applyShaders(announce: true);
+      expect(await mpv.getProperty('glsl-shaders'), '');
+      expect(p.shaderNotice.value?.text,
+          'Custom shaders failed: "gone.glsl" not found — turned off');
+      expect(s.shaderPreset.value, ShaderPreset.off);
+
+      // Custom files that exist.
+      s.setCustomShaders([good.path]);
+      s.setShaderPreset(ShaderPreset.custom);
+      await p.applyShaders();
+      expect(await mpv.getProperty('glsl-shaders'), good.path);
+
+      // A recreated player (Reload video) gets the choice again.
+      await p.recreate();
+      expect(await (p.player.platform as dynamic).getProperty('glsl-shaders'),
+          good.path);
+
+      s.setShaderPreset(ShaderPreset.off);
+      await p.applyShaders(announce: true);
+      expect(await (p.player.platform as dynamic).getProperty('glsl-shaders'),
+          '');
+      expect(p.shaderNotice.value?.text, 'Shaders: Off');
+    } finally {
+      await p.dispose();
+    }
+  });
+}