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/package.json

@@ -8,7 +8,8 @@
     "build": "vite build",
     "preview": "vite preview",
     "lint": "echo 'Linting passed'",
-    "test:run": "echo 'Frontend checks passed'"
+    "test": "vitest",
+    "test:run": "vitest run"
   },
   "dependencies": {
     "@types/prismjs": "^1.26.6",
@@ -19,13 +20,17 @@
     "react-dom": "^18.3.1"
   },
   "devDependencies": {
+    "@testing-library/dom": "^10.4.2",
+    "@testing-library/react": "^16.3.3",
     "@types/react": "^18.3.12",
     "@types/react-dom": "^18.3.1",
     "@vitejs/plugin-react": "^4.3.4",
     "autoprefixer": "^10.4.20",
+    "jsdom": "^25.0.1",
     "postcss": "^8.4.49",
     "tailwindcss": "^3.4.17",
     "typescript": "^5.6.3",
-    "vite": "^5.4.11"
+    "vite": "^5.4.11",
+    "vitest": "^2.1.9"
   }
 }