Replace true/false icons with T/F letters for better readability

[Copilot]

This PR addresses user feedback about difficulty understanding true/false questions after flipping cards. The previous implementation used CheckCircle (✓) and XCircle (✗) icons from lucide-react, which users found less intuitive than simple text letters.

Changes Made

• Replaced CheckCircle and XCircle icons with simple "T" and "F" text
• Removed unused lucide-react imports
• Added appropriate styling (text-lg font-semibold select-none) for clear text display
• Maintained all existing logic and functionality

UI Comparison

Benefits

The new text-based approach provides:

• Clearer meaning: "T" and "F" are universally understood as True/False
• Better readability: Text is more immediately recognizable than abstract icons
• Improved accessibility: Screen readers can better interpret text content
• Language neutrality: T/F are recognized across different educational systems

Technical Details

The change is minimal and surgical - only the visual representation is modified while preserving all existing behavior. The T/F letters represent the user's chosen answer (if they answered) or the correct answer (if they didn't answer), maintaining the current logic.

All tests pass and the code lints cleanly with no breaking changes.

Fixes #145.

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