chore(deps): upgrade to React 19

[PierreRaybaut]

Replaces #45 and #46: react, react-dom and their type packages must move together (each half fails with ERESOLVE alone).

• @types/react 19 drops the global JSX namespace: four components now return ReactElement.
• React 19's react-dom adds ~23 KiB gzip to the initial chunk (react-vendor 45.5 -> 68.4 KiB), so the initial JS budget goes from 375 to 400 KiB (now 387.3 KiB).
• Dependabot now groups react, react-dom, @types/react and @types/react-dom so their majors arrive in one PR.

tsc, ESLint, Prettier, Vitest (745), build with bundle budget and the smoke, signal-layout, image-pan, macro, notebooks and worker_mode Playwright specs (23) pass locally.