feat: support React 19 while preserving React 18 consumers

[codeacme17]

What to build

Modernize Echo UI development and documentation to React 19 without unnecessarily dropping applications that still use React 18. Library consumers should receive one compatible package, clear peer requirements, and tested behavior on both supported React generations.

Acceptance criteria

• Development, examples, and Nextra documentation run on React and React DOM 19 with matching types.
• React 19 type and runtime changes affecting refs, callbacks, Strict Mode, and error handling are addressed.
• Published peer ranges accurately include the supported React 18 and React 19 versions.
• Consumer smoke tests install and render the built package with React 18.3 and the selected React 19 release.
• Public component and Hook behavior remains compatible unless a change is explicitly documented.

Blocked by

• build: migrate type checking and linting to TypeScript 6 and ESLint 10 #77 — build: migrate type checking and linting to TypeScript 6 and ESLint 10.
• build: upgrade the library build and test stack to Vite 8 and Vitest 4 #78 — build: upgrade the library build and test stack to Vite 8 and Vitest 4.
• docs: cut over to Nextra and remove IslandJS #84 — docs: cut over to Nextra and remove IslandJS.