feat: replace screen state machine with React Router v7 URL routing

- Install react-router-dom
- Add slugify utility with full TDD test suite (7 tests)
- Wrap app in BrowserRouter in main.tsx
- Add optional onNavigate prop to ExamRunner.goTo
- Create ExamRoute component (deep-link restore + back/forward sync)
- Migrate App.tsx: replace screen useState with <Routes>, all setScreen()
  calls replaced with navigate(), add activateExam for deep-link activation,
  use useLocation for verify/reset-password special layouts
- Remove unused AppScreen type from types.ts

frontend/package.json

@@ -15,7 +15,8 @@
     "dompurify": "^3.4.1",
     "katex": "^0.16.45",
     "react": "^19.2.5",
-    "react-dom": "^19.2.5"
+    "react-dom": "^19.2.5",
+    "react-router-dom": "^7.14.2"
   },
   "devDependencies": {
     "@babel/core": "^7.29.0",