TP-27 — Startup fallback for missing/broken selected theme

[ZhuchkaTriplesix]

Epic: E
Depends on: TP-17

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

Goal

Prevent broken custom themes from breaking app startup.

Files

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

Implementation

On ThemeController.load():

1. Read selected theme id/path.
2. Try registry load.
3. If failure:
   • set active theme to Querya Dark;
   • keep selectedThemeLoadError;
   • do not crash;
   • do not delete user setting automatically.
4. Preferences can show:
   • "Selected theme failed to load. Using Querya Dark."

Acceptance Criteria

• Missing selected file starts app with Querya Dark.
• Invalid selected file starts app with Querya Dark.
• Error visible in Preferences.
• User can choose another theme and clear error.

Tests

• missing file;
• invalid file;
• subsequent valid selection clears error.

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