docs: say component tests are allowed, not absent by rule

CLAUDE.md read as a prohibition — "Vitest for its own pure helpers only, no
React, no DOM". It was only ever a description of what was installed, and it
was talking future contributors out of a test they are allowed to write.

Tests now states what exists, that component tests are allowed, and what the
first one costs: jsdom or happy-dom, @testing-library/react v16+ for React 19,
environment and a *.spec.tsx include. Notes that babel-plugin-react-compiler is
in devDependencies but not enabled in next.config.ts, so there is no transform
mismatch to work around.

Adds which to reach for. A decision belongs in a pure test, extracted into
components/{feature}/. A defect only a real render shows — ref-versus-state
timing, effect ordering — belongs in a component test, because no pure test can
see it. VoiceReviewSheet is the cheap first one; TreatmentWorkspace needs six
axios mocks and earns its keep only for a bug that needs it.

Same correction in vitest.config.ts's docblock and the spec's §12 line, which
each carried their own copy of "no React, no DOM".

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/vitest.config.ts

@@ -2,8 +2,11 @@ import path from 'node:path';
 import { defineConfig } from 'vitest/config';
 
 /**
- * Covers the pure helpers only — no React, no DOM. `@/*` mirrors `tsconfig.json`'s path so a
- * spec can import the same modules the app does.
+ * Covers the pure helpers, which is all that exists today — hence no `environment` and a
+ * `*.spec.ts` include. Component tests are not ruled out: add jsdom, @testing-library/react,
+ * `environment: 'jsdom'` and `*.spec.tsx` here. See CLAUDE.md > Tests.
+ *
+ * `@/*` mirrors `tsconfig.json`'s path so a spec can import the same modules the app does.
  */
 export default defineConfig({
   resolve: {