TP-15 — Persist selected theme id/path in AppSettings

[ZhuchkaTriplesix]

Epic: B
Depends on: TP-10

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

Goal

Persist selected registry theme across restarts without storing heavy objects.

Files

• lib/core/storage/app_settings.dart
• test/core/storage/app_settings_test.dart

Implementation

Add keys:

• theme_selected_id
• theme_selected_source
• theme_selected_path

Add methods:

Future<String?> getSelectedThemeId();
Future<void> setSelectedThemeId(String? id);
Future<String?> getSelectedThemeSource();
Future<void> setSelectedThemeSource(String? source);
Future<String?> getSelectedThemePath();
Future<void> setSelectedThemePath(String? path);

Keep existing preset/imported settings unchanged.

Acceptance Criteria

• Settings roundtrip.
• Clearing selected theme works.
• No SQL workspace revision bump unless existing theme settings already do that intentionally.

Tests

• id/source/path roundtrip;
• clear values;
• existing theme preset tests still pass.

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