feat: implement universal icon system with Phosphor Icons
- Install @phosphor-icons/react package
- Create centralized Icon component with size/weight/color variants
- Create icon registry with 34 icons across 5 categories
- Replace all raw Unicode symbols with proper icon components
- Add icons to navigation, buttons, status indicators, git operations
- Add icon CSS with consistent sizing and spacing
- Fix type definitions for Phosphor icon compatibility
Quality gates: typecheck ✓, lint ✓, build ✓ (375KB bundle)
Fusion
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