TP-10 — Add `ThemeDefinition`

[ZhuchkaTriplesix]

Epic: B
Depends on: TP-03

Source: docs/theme-parser-github-issues.md

Goal

Represent lightweight theme metadata for lists/pickers without full parsing.

Files

• lib/core/theme/theme_definition.dart
• test/core/theme/theme_definition_test.dart

Implementation

Add:

enum ThemeSource { builtin, imported, filesystem, legacyImported }
enum ThemeFormat { queryaCustom, vscode }

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,
  });
}

Add helpers:

• bool get isFileBacked
• String get stableCacheKey

Acceptance Criteria

• ThemeDefinition is immutable.
• stableCacheKey changes when contentHash changes.
• Does not depend on Flutter widgets.

Tests

• file-backed vs builtin;
• cache key includes id/hash/source;
• equality if implemented.

---