TP-20 — Add safe preview card without applying theme on hover

[ZhuchkaTriplesix]

Epic: C
Depends on: TP-18, TP-17

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

Goal

Optional visual preview for hovered/selected theme without rebuilding the whole app.

Files

• lib/features/settings/theme_preview_card.dart
• lib/features/settings/theme_picker_button.dart
• tests as needed.

Implementation

Add ThemePreviewCard:

• accepts QueryaTheme or lightweight preview colors;
• displays:
  • background;
  • surface;
  • primary/accent;
  • sample text;
  • editor background strip.

In picker:

• hover selects preview target id locally;
• debounce 100-150ms before calling previewThemeById;
• preview result stored in local state only;
• never call setThemeById on hover.

Acceptance Criteria

• Hovering row does not change app theme.
• Preview card updates after debounce.
• Broken preview shows non-blocking error in card.

Tests

• hover/callback does not call onSelected;
• preview future resolves and card updates;
• broken preview shows fallback/error.

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