[RyPope]

Moves react, react-dom, @types/react and @types/react-dom to 19.3 together. Dependabot proposed these separately (#11, #19), which can't work: react-dom 19 requires react 19.

Breaking changes handled

• useRef requires an argument in React 19's types: useRef<DraftSaver | undefined>(undefined) in useWorkspace.
• Nothing else in the app uses APIs removed in 19 (no ReactDOM.render, defaultProps, propTypes, string refs or legacy context). The entry point already uses createRoot.

Libraries checked

react-router-dom 7, @tanstack/react-query 5, and @monaco-editor/react 4.7 all support React 19.

Verification

• npm run typecheck, npm test (17 passed), npm run lint, npm run build.
• Manually against a running stack: problem list, workspace (editor, Run, Submit accepted 71/71), Solutions tab with Open in editor, and the Progress page. No React warnings or errors in the console.

The toolchain upgrade (TypeScript, Vite, Vitest, ESLint) follows in a separate PR stacked on this one.