fix: include loaded paths in viewBox

[w3lld1]

Summary

• expand the withViewBox bounds to include loaded path coordinates
• account for stroke radius so off-canvas stroke edges remain visible
• add unit and browser coverage for paths extending beyond the rendered canvas

Validation

• pnpm lint
• pnpm test:unit (89 tests)
• pnpm --filter react-sketch-canvas exec playwright test -c playwright-ct.config.ts --workers=1 (144 tests across Chromium and Firefox)
• pnpm --filter react-sketch-canvas build
• E2E: 14/16 passed; the existing 500-stroke × 500-point stress case exceeded its 10-second timeout in Chromium and Firefox on both this branch and an unchanged main worktree in this runner

Fixes #212