feat: interactive force-directed knowledge graph visualization

Canvas-based graph with force simulation:
- Nodes colored by type (concept=gold, file=blue, function=teal,
  error=red, pattern=purple, library=pink, etc.)
- Node size proportional to connection count
- Repulsion + attraction physics with damping
- Pan (click+drag), zoom (scroll wheel), reset button
- Hover tooltip showing type, name, connection count
- Color legend bar
- Stats (nodes/edges) + breakdown tables below canvas
- GET /imprint/graph/all endpoint returns all nodes+edges

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JohnPitter