fix(jsoncrack-react): reliable fit-to-center and responsive viewport

- Fix off-center auto-fit on initial load and on manual Fit clicks
- Keep the viewport in sync with host container resizes
- Force a sans-serif font on edge labels so host-page CSS cannot override
- Add role="img" and aria-label on the canvas wrapper
- Narrow `json` prop type from `string | object | unknown[]` to `string | object`
- Extract pure helpers into canvasHelpers.ts with JSDoc
- Add vitest suites for parseGraph and canvasHelpers (29 tests)

Bumps version to 1.0.2 and adds CHANGELOG.md.