Test infra: jsdom environment + .test.tsx support so hooks and components are testable

[Rinava]

Problem

vitest.config.ts:6-7 sets environment: "node" and include: ["src/**/*.test.ts"]. Two consequences:

1. Every hook (use-local-storage, use-debounced-value, use-contributors) and component is structurally untestable — no jsdom, no Testing Library.
2. The .ts-only glob is a trap: a contributor who adds a .test.tsx gets a green run that silently never executed their file.

The pure src/lib logic is well covered (86 tests) — this is specifically about the UI half of the codebase.

Proposed solution

• Split Vitest into projects (or use environmentMatchGlobs): node for src/lib, jsdom for hooks/components.
• Add jsdom + @testing-library/react as devDependencies.
• Broaden the include to src/**/*.test.{ts,tsx}.
• Land one seed test (e.g. use-debounced-value with fake timers) proving the pipeline.

Acceptance criteria

• A .test.tsx file actually runs.
• The seed hook test passes; the existing 86 tests stay green.