Migrate to React 19

[gabitoesmiapodo]

User story / Problem statement

Currently React is a full major behind (18.3.1 vs 19.x). The longer the gap, the higher the migration cost. This is the headline migration of the parent epic.

Expected outcome

canton-connect-kit, carpincho-wallet, and dapp/frontend build and pass on React 19, with the connect-kit peer range accepting React 19.

Acceptance criteria

• react + react-dom 18.3.1 → 19.x (all three packages)
• @types/react 18 → 19 and @types/react-dom 18 → 19 in lockstep
• Widen canton-connect-kit peerDependencies.react to accept 19 (^18 || ^19 or ^19)
• Address React 19 removals at call sites (legacy context, string refs, defaultProps on functions, ReactDOM.render, etc.)
• Lint + typecheck + unit tests + e2e pass

Technical notes

Part of #76. The four React-related packages must move together; stricter 19 typings will surface new type errors.