TP-24 — Add built-in theme assets

[ZhuchkaTriplesix]

Epic: D
Depends on: TP-12

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

Goal

Ship built-in sample themes in release builds, not only as repository files.

Files

• assets/themes/
• pubspec.yaml
• lib/core/theme/theme_registry_service.dart
• tests as feasible.

Implementation

Move/copy curated themes to:

• assets/themes/cyberpunk-neon.json
• any other approved built-in themes.

Update pubspec.yaml:

flutter:
  assets:
    - assets/themes/

Registry:

• load built-in asset manifest;
• create ThemeDefinition(source: ThemeSource.builtin);
• load full theme from asset when selected.

Acceptance Criteria

• Built-in themes show in picker in release/profile builds.
• App does not depend on repo themes/samples/ path at runtime.
• Existing themes/samples/ can remain for docs/manual testing.

Tests

• Asset loading if test environment supports bundle.
• Otherwise unit-test parsing with same file content.

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