feat(paint): classic bitmap editor with tools, undo, and PNG export

Summary

• Adds src/paint/ — MS Paint-inspired bitmap editor (pencil, brush, eraser, line, rect, ellipse, fill, undo, PNG export)
• Original code only; DOCUMENTED comparison notes in app README
• Amplify + customHttp + root README Creative Tools row; squircle icons from master asset via sharp

Test plan

• cd src/paint && npm ci && npm test && npm run build
• Smoke: draw with each tool, undo, export PNG