refactor: migrate all Lucide icons to Phosphor + remove lucide-react (PR4)
Unified icon library:
- Create src/components/ui/icon.tsx as centralized icon re-export + size constants
- Migrate all 20+ files from lucide-react to @phosphor-icons/react
- Remove lucide-react from package.json dependencies
- Upgrade ESLint lucide-react rule from warn → error
Icon mappings applied:
CheckIcon → Check, ChevronDown → CaretDown, ChevronUp → CaretUp,
ChevronRight → CaretRight, X → X, Search → MagnifyingGlass,
Loader2 → SpinnerGap, CornerDownLeft → ArrowElbowDownLeft,
Image → Image, Brain → Brain, Terminal → Terminal, Book → Book,
Dot → DotOutline, Circle → Circle
Files migrated: ui/ (6), ai-elements/ (11), layout/ (3)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
