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>

frontend/vitest.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config';
+import react from '@vitejs/plugin-react';
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    setupFiles: ['./src/test/setup.ts'],
+  },
+});