TP-22 — Add refresh themes action in Preferences

[ZhuchkaTriplesix]

Epic: C
Depends on: TP-17, TP-21

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

Goal

Let users refresh filesystem themes without restarting the app.

Files

• lib/features/settings/preferences_appearance_section.dart
• lib/core/theme/theme_controller.dart

Implementation

Add button near import/reset:

• Refresh themes
• calls ThemeController.loadAvailableThemes();
• shows small loading state;
• preserves active theme if still available;
• if active theme changed on disk, optionally reload when selected again, not immediately.

Acceptance Criteria

• Refresh updates list.
• Broken files do not break Preferences.
• Loading state does not block whole dialog.

Tests

• fake controller list changes after refresh;
• button disabled while refreshing.

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