Add PracticePreview component that allows practicing any problem while
in browse mode without affecting session state. The practice panel
displays inline below the problem card with a clear header indicating
it doesn't affect the session, preventing UX confusion.
- Add PracticePreview component with keyboard and numpad input support
- Add inline mode to PracticePreview for embedded display
- Update BrowseModeView to show practice panel below problem card
- Toggle button switches between "Practice This Problem" / "Close Practice Panel"
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Co-Authored-By: Claude <noreply@anthropic.com>
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