TP-17 — Integrate registry into ThemeController load

[ZhuchkaTriplesix]

Epic: B
Depends on: TP-13, TP-15, TP-16

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

Goal

Make ThemeController aware of registry themes while preserving existing presets.

Files

• lib/core/theme/theme_controller.dart
• lib/core/theme/querya_theme_preset.dart
• tests for theme controller.

Implementation

Add state:

• _availableThemes: List<ThemeDefinition>
• _selectedThemeId: String?
• _selectedThemePath: String?
• _selectedThemeLoadError: String?

Add getters:

• availableThemes
• selectedThemeId
• selectedThemeLoadError

Add methods:

Future<void> loadAvailableThemes();
Future<void> setThemeById(String id);
Future<ThemeLoadResult> previewThemeById(String id);

Behavior:

• load() first loads existing mode/preset.
• Then load registry definitions async.
• If stored selected id exists, try load it.
• On failure, apply Querya Dark fallback but keep error visible.

Performance Notes

• Do not call notifyListeners() for every discovered file.
• Batch registry load and notify once.
• previewThemeById must not mutate active app theme.

Acceptance Criteria

• Existing setPreset() behavior still works.
• New setThemeById() applies registry theme.
• Broken selected theme falls back to Querya Dark.
• previewThemeById() returns theme/result without notifying app listeners.

Tests

• old presets still pass;
• select by id persists setting;
• preview does not change activeTheme;
• broken selected id fallback.

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