feat(theme): ThemeLoadResult sealed types (#104)

lib/core/theme/theme_definition.dart

@@ -0,0 +1,42 @@
+enum ThemeSource {
+  builtin,
+  imported,
+  filesystem,
+  legacyImported,
+}
+
+enum ThemeFormat {
+  queryaCustom,
+  vscode,
+}
+
+/// Lightweight theme metadata for registry lists and load results.
+class ThemeDefinition {
+  const ThemeDefinition({
+    required this.id,
+    required this.name,
+    required this.source,
+    required this.format,
+    required this.isDark,
+    this.path,
+    this.lastModified,
+    this.contentHash,
+  });
+
+  final String id;
+  final String name;
+  final ThemeSource source;
+  final ThemeFormat format;
+  final bool isDark;
+  final String? path;
+  final DateTime? lastModified;
+  final String? contentHash;
+
+  bool get isFileBacked =>
+      source == ThemeSource.filesystem ||
+      source == ThemeSource.imported ||
+      source == ThemeSource.legacyImported;
+
+  String get stableCacheKey =>
+      '${source.name}:$id:${contentHash ?? path ?? name}';
+}

lib/core/theme/theme_load_result.dart

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+import 'querya_theme.dart';
+import 'theme_definition.dart';
+
+/// Result of loading a [ThemeDefinition] into a runtime [QueryaTheme].
+sealed class ThemeLoadResult {
+  const ThemeLoadResult();
+}
+
+class ThemeLoadSuccess extends ThemeLoadResult {
+  const ThemeLoadSuccess({
+    required this.definition,
+    required this.theme,
+  });
+
+  final ThemeDefinition definition;
+  final QueryaTheme theme;
+}
+
+class ThemeLoadFailure extends ThemeLoadResult {
+  const ThemeLoadFailure({
+    required this.definition,
+    required this.message,
+    this.error,
+  });
+
+  final ThemeDefinition definition;
+  final String message;
+  final Object? error;
+}

test/core/theme/theme_load_result_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:querya_desktop/core/theme/querya_theme.dart';
+import 'package:querya_desktop/core/theme/theme_definition.dart';
+import 'package:querya_desktop/core/theme/theme_load_result.dart';
+
+void main() {
+  const definition = ThemeDefinition(
+    id: 'fixture-custom-dark',
+    name: 'Fixture Custom Dark',
+    source: ThemeSource.filesystem,
+    format: ThemeFormat.queryaCustom,
+    isDark: true,
+    path: '/tmp/fixture-custom-dark.json',
+    contentHash: 'abc123',
+  );
+
+  group('ThemeLoadResult', () {
+    test('success carries definition and theme', () {
+      const result = ThemeLoadSuccess(
+        definition: definition,
+        theme: QueryaTheme.darkDefault,
+      );
+
+      expect(result, isA<ThemeLoadSuccess>());
+      expect(result.definition.id, 'fixture-custom-dark');
+      expect(result.theme.brightness, QueryaTheme.darkDefault.brightness);
+    });
+
+    test('failure carries definition, message, and optional error', () {
+      final error = StateError('parse failed');
+      final result = ThemeLoadFailure(
+        definition: definition,
+        message: 'Invalid theme file.',
+        error: error,
+      );
+
+      expect(result, isA<ThemeLoadFailure>());
+      expect(result.definition.path, definition.path);
+      expect(result.message, 'Invalid theme file.');
+      expect(result.error, same(error));
+    });
+
+    test('sealed result supports pattern matching', () {
+      const ThemeLoadResult result = ThemeLoadSuccess(
+        definition: definition,
+        theme: QueryaTheme.lightDefault,
+      );
+
+      final matched = switch (result) {
+        ThemeLoadSuccess(:final theme) => theme.brightness,
+        ThemeLoadFailure() => null,
+      };
+
+      expect(matched, QueryaTheme.lightDefault.brightness);
+    });
+  });
+}