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/src/test/setup.ts

@@ -0,0 +1,9 @@
+import { afterEach } from 'vitest';
+import { cleanup } from '@testing-library/react';
+
+// jsdom has no layout engine, so scrolling is a no-op here
+Element.prototype.scrollIntoView = () => {};
+
+afterEach(() => {
+  cleanup();
+});