Replace 3-way mode tabs (Smart/Manual/Mastery) with unified interface:
- Smart and Mastery are now "quick presets" via 2-tab selector
- Manual controls (thermometers) always visible for customization
- All modes share same displayRules system (1:1 correspondence)
Changes:
- Add DifficultyMethodSelector component with tab-style UI
- Update ConfigPanel to use 2-tab selector
- Smart/Mastery populate the always-visible display controls
- Preserve displayRules when switching between methods
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
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