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)

openspec/changes/universal-icon-system/proposal.md

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+# Universal Icon System
+
+## Problem
+
+The current project uses inconsistent icon implementations across the frontend:
+- Raw Unicode symbols (✓, ✗, ⚠, etc.) which render differently across browsers and operating systems
+- No standardized icon component or library
+- Inconsistent visual language throughout the UI
+- Some icons may not render at all on certain systems
+
+This creates a fragmented user experience and potential accessibility issues.
+
+## Solution
+
+Implement a universal icon system using **Phosphor Icons** - a comprehensive, lightweight icon library designed for modern web applications:
+
+1. **Consistent rendering** across all browsers and platforms
+2. **Comprehensive icon set** with 7000+ icons covering all use cases
+3. **Multiple weights** (thin, light, regular, bold, fill, duotone) for flexibility
+4. **Tree-shakeable** - only includes icons that are actually used
+5. **React integration** with phosphor-react library
+6. **Accessible** with proper ARIA labels and focus management
+
+## Key Features
+
+### Icon Component
+- Centralized `<Icon>` component wrapping Phosphor icons
+- Consistent sizing (sm, md, lg, xl)
+- Color inheritance from parent or explicit color prop
+- Accessibility support (aria-label, role)
+
+### Icon Categories
+- Navigation (home, settings, user, etc.)
+- Actions (edit, delete, save, add, etc.)
+- Status (success, error, warning, info)
+- Files and folders
+- Git operations (branch, commit, merge, etc.)
+- Development tools (terminal, code, database, etc.)
+
+### Migration Strategy
+- Replace all raw Unicode symbols with proper icon components
+- Update existing components to use the new icon system
+- Maintain visual consistency during migration
+
+## Benefits
+
+- **Cross-browser consistency** - Icons render identically everywhere
+- **Better accessibility** - Screen reader friendly with proper labels
+- **Improved maintainability** - Single source of truth for icons
+- **Enhanced UX** - Professional, polished appearance
+- **Future-proof** - Easy to add new icons as needed
+
+## Success Criteria
+
+- [ ] All raw Unicode symbols replaced with icon components
+- [ ] Consistent icon sizing and styling across all pages
+- [ ] No visual regressions in existing UI
+- [ ] Icons render correctly in all supported browsers