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/vite.config.ts

@@ -1,23 +1,19 @@
-import { defineConfig } from 'vitest/config'
-import react, { reactCompilerPreset } from '@vitejs/plugin-react'
-import babel from '@rolldown/plugin-babel'
+import { defineConfig } from "vitest/config";
+import react, { reactCompilerPreset } from "@vitejs/plugin-react";
+import babel from "@rolldown/plugin-babel";
 
 export default defineConfig({
-  plugins: [
-    react(),
-    babel({ presets: [reactCompilerPreset()] })
-  ],
+  plugins: [react(), babel({ presets: [reactCompilerPreset()] })],
   server: {
     proxy: {
-      '/api': {
-        target: 'http://localhost:3001',
+      "/api": {
+        target: "http://localhost:3001",
         changeOrigin: true,
       },
     },
   },
   test: {
-    environment: 'node',
+    environment: "node",
     globals: true,
   },
-})
-
+});