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/src/main.tsx

@@ -1,11 +1,14 @@
-import { StrictMode } from 'react'
-import { createRoot } from 'react-dom/client'
-import 'katex/dist/katex.min.css'
-import './index.css'
-import App from './App.tsx'
+import { StrictMode } from "react";
+import { createRoot } from "react-dom/client";
+import { BrowserRouter } from "react-router-dom";
+import "katex/dist/katex.min.css";
+import "./index.css";
+import App from "./App.tsx";
 
-createRoot(document.getElementById('root')!).render(
+createRoot(document.getElementById("root")!).render(
   <StrictMode>
-    <App />
+    <BrowserRouter>
+      <App />
+    </BrowserRouter>
   </StrictMode>,
-)
+);