TP-16 — Migrate legacy imported theme into registry

[ZhuchkaTriplesix]

Epic: B
Depends on: TP-12, TP-15

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

Goal

Users with existing imported VS Code themes should keep them after registry lands.

Files

• lib/core/theme/theme_registry_service.dart
• lib/core/theme/theme_import_service.dart
• lib/core/theme/theme_controller.dart
• tests as needed.

Implementation

During registry load:

• check existing persisted import path/name/colors;
• if found, add a ThemeDefinition:
  • id: legacy-imported or imported;
  • source: legacyImported;
  • format: vscode;
  • path: stored import file;
  • name: importedThemeName ?? "Imported theme".

Do not delete old settings.

Acceptance Criteria

• Existing QueryaThemePreset.imported still applies.
• Legacy imported theme appears in new picker.
• Missing legacy file falls back gracefully.

Tests

• fake old imported path -> registry definition exists;
• selected legacy imported theme loads;
• missing old file does not crash.

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