test(frontend): set up Vitest with jsdom and run tests in the image build

- Add vitest, jsdom and Testing Library as dev dependencies,
  replace the placeholder test:run script with vitest run.
- Add outliner tests for Backspace merging and caret placement on click.
- Run the frontend tests in the Dockerfile before the Vite build.
- Update docs (README, architecture, FLAW-19).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -6,7 +6,7 @@ WORKDIR /app/frontend
 COPY frontend/package*.json ./
 RUN npm install --no-audit --no-fund
 COPY frontend/ ./
-RUN npx tsc --noEmit && npm run build
+RUN npx tsc --noEmit && npm run test:run && npm run build
 
 # ==============================================================================
 # Stage 2: Go Backend Lint, Unit Tests & Binary Build (Cached Go module & sqlite layer)