refactor(App): migrate to Data Router for improved routing and unsaved changes handling

- Replaced `BrowserRouter` and `Routes` with `createBrowserRouter` and `RouterProvider` to support unsaved changes blocking.
- Restructured route definitions for better organization and clarity, maintaining existing functionality.
- Added comments to clarify the necessity of the Data Router for handling unsaved changes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.jsx

@@ -1,8 +1,7 @@
 import React from 'react'
 import {
-  BrowserRouter as Router,
-  Routes,
-  Route,
+  RouterProvider,
+  createBrowserRouter,
   Navigate,
   NavLink,
   useLocation,
@@ -163,115 +162,115 @@ function PublicRoute({ children }) {
   return !isAuthenticated ? children : <Navigate to="/" replace />
 }
 
-function AppRoutes() {
-  return (
-    <Routes>
-      <Route path="/verify" element={<VerifyPage />} />
-
-      <Route
-        path="/login"
-        element={
-          <PublicRoute>
-            <LoginPage />
-          </PublicRoute>
-        }
-      />
-
-      {/* P-01: Öffentliche Rechtstextseiten — kein Auth erforderlich */}
-      <Route path="/impressum" element={<LegalPage type="impressum" />} />
-      <Route path="/datenschutz" element={<LegalPage type="datenschutz" />} />
-      <Route path="/nutzungsbedingungen" element={<LegalPage type="nutzungsbedingungen" />} />
-      <Route path="/medienrichtlinie" element={<LegalPage type="medienrichtlinie" />} />
-
-      <Route element={<ProtectedLayout />}>
-        <Route index element={<Dashboard />} />
-        <Route path="profile" element={<Navigate to="/settings" replace />} />
-        <Route path="settings" element={<AccountSettingsPage />} />
-        <Route path="settings/system" element={<SettingsSystemInfoPage />} />
-        <Route path="settings/legal" element={<SettingsLegalPage />} />
-        <Route path="media" element={<MediaLibraryPage />} />
-        <Route path="exercises">
-          <Route index element={<ExercisesListPage />} />
-          <Route path="new" element={<ExerciseFormPage />} />
-          <Route path=":id/edit" element={<ExerciseFormPage />} />
-          <Route path=":id" element={<ExerciseDetailPage />} />
-        </Route>
-        <Route path="clubs" element={<ClubsPage />} />
-        <Route path="inbox" element={<InboxPage />} />
-        <Route path="skills" element={<SkillsPage />} />
-        <Route path="planning/framework-programs/new" element={<TrainingFrameworkProgramEditPage />} />
-        <Route path="planning/framework-programs/:id" element={<TrainingFrameworkProgramEditPage />} />
-        <Route path="planning/framework-programs" element={<TrainingFrameworkProgramsListPage />} />
-        <Route path="planning/training-modules/new" element={<TrainingModuleEditPage />} />
-        <Route path="planning/training-modules/:id" element={<TrainingModuleEditPage />} />
-        <Route path="planning/training-modules" element={<TrainingModulesListPage />} />
-        <Route path="planning/run/:unitId/coach" element={<TrainingCoachPage />} />
-        <Route path="planning/run/:unitId" element={<TrainingUnitRunPage />} />
-        <Route path="planning" element={<TrainingPlanningPage />} />
-        <Route path="admin" element={<AdminHomeRedirect />} />
-        <Route
-          path="admin/users"
-          element={
-            <PlatformAdminRoute>
-              <AdminUsersPage />
-            </PlatformAdminRoute>
-          }
-        />
-        <Route
-          path="admin/hierarchy"
-          element={
-            <PlatformAdminRoute>
-              <AdminHierarchyPage />
-            </PlatformAdminRoute>
-          }
-        />
-        <Route
-          path="admin/maturity-models"
-          element={
-            <PlatformAdminRoute>
-              <AdminMaturityModelsPage />
-            </PlatformAdminRoute>
-          }
-        />
-        <Route
-          path="admin/catalogs"
-          element={
-            <PlatformAdminRoute>
-              <AdminCatalogsPage />
-            </PlatformAdminRoute>
-          }
-        />
-        <Route
-          path="admin/mediawiki-import"
-          element={
-            <PlatformAdminRoute>
-              <MediaWikiImportPage />
-            </PlatformAdminRoute>
-          }
-        />
-        <Route
-          path="admin/legal-documents"
-          element={
-            <PlatformAdminRoute>
-              <AdminLegalDocumentsPage />
-            </PlatformAdminRoute>
-          }
-        />
-        <Route path="trainer-contexts" element={<TrainerContextsPage />} />
-      </Route>
-
-      <Route path="*" element={<Navigate to="/" replace />} />
-    </Routes>
-  )
-}
+/**
+ * Data Router — erforderlich für `useBlocker` (ungespeicherte Änderungen).
+ * Klassisches `BrowserRouter` stellt keinen DataRouterContext bereit; ohne Migration
+ * werfen Seiten mit `useUnsavedChangesBlocker` beim Rendern eine Invariante.
+ */
+const appRouter = createBrowserRouter([
+  { path: '/verify', element: <VerifyPage /> },
+  {
+    path: '/login',
+    element: (
+      <PublicRoute>
+        <LoginPage />
+      </PublicRoute>
+    ),
+  },
+  { path: '/impressum', element: <LegalPage type="impressum" /> },
+  { path: '/datenschutz', element: <LegalPage type="datenschutz" /> },
+  { path: '/nutzungsbedingungen', element: <LegalPage type="nutzungsbedingungen" /> },
+  { path: '/medienrichtlinie', element: <LegalPage type="medienrichtlinie" /> },
+  {
+    element: <ProtectedLayout />,
+    children: [
+      { index: true, element: <Dashboard /> },
+      { path: 'profile', element: <Navigate to="/settings" replace /> },
+      { path: 'settings', element: <AccountSettingsPage /> },
+      { path: 'settings/system', element: <SettingsSystemInfoPage /> },
+      { path: 'settings/legal', element: <SettingsLegalPage /> },
+      { path: 'media', element: <MediaLibraryPage /> },
+      {
+        path: 'exercises',
+        children: [
+          { index: true, element: <ExercisesListPage /> },
+          { path: 'new', element: <ExerciseFormPage /> },
+          { path: ':id/edit', element: <ExerciseFormPage /> },
+          { path: ':id', element: <ExerciseDetailPage /> },
+        ],
+      },
+      { path: 'clubs', element: <ClubsPage /> },
+      { path: 'inbox', element: <InboxPage /> },
+      { path: 'skills', element: <SkillsPage /> },
+      { path: 'planning/framework-programs/new', element: <TrainingFrameworkProgramEditPage /> },
+      { path: 'planning/framework-programs/:id', element: <TrainingFrameworkProgramEditPage /> },
+      { path: 'planning/framework-programs', element: <TrainingFrameworkProgramsListPage /> },
+      { path: 'planning/training-modules/new', element: <TrainingModuleEditPage /> },
+      { path: 'planning/training-modules/:id', element: <TrainingModuleEditPage /> },
+      { path: 'planning/training-modules', element: <TrainingModulesListPage /> },
+      { path: 'planning/run/:unitId/coach', element: <TrainingCoachPage /> },
+      { path: 'planning/run/:unitId', element: <TrainingUnitRunPage /> },
+      { path: 'planning', element: <TrainingPlanningPage /> },
+      { path: 'admin', element: <AdminHomeRedirect /> },
+      {
+        path: 'admin/users',
+        element: (
+          <PlatformAdminRoute>
+            <AdminUsersPage />
+          </PlatformAdminRoute>
+        ),
+      },
+      {
+        path: 'admin/hierarchy',
+        element: (
+          <PlatformAdminRoute>
+            <AdminHierarchyPage />
+          </PlatformAdminRoute>
+        ),
+      },
+      {
+        path: 'admin/maturity-models',
+        element: (
+          <PlatformAdminRoute>
+            <AdminMaturityModelsPage />
+          </PlatformAdminRoute>
+        ),
+      },
+      {
+        path: 'admin/catalogs',
+        element: (
+          <PlatformAdminRoute>
+            <AdminCatalogsPage />
+          </PlatformAdminRoute>
+        ),
+      },
+      {
+        path: 'admin/mediawiki-import',
+        element: (
+          <PlatformAdminRoute>
+            <MediaWikiImportPage />
+          </PlatformAdminRoute>
+        ),
+      },
+      {
+        path: 'admin/legal-documents',
+        element: (
+          <PlatformAdminRoute>
+            <AdminLegalDocumentsPage />
+          </PlatformAdminRoute>
+        ),
+      },
+      { path: 'trainer-contexts', element: <TrainerContextsPage /> },
+    ],
+  },
+  { path: '*', element: <Navigate to="/" replace /> },
+])
 
 function App() {
   return (
     <AuthProvider>
       <ToastProvider>
-        <Router>
-          <AppRoutes />
-        </Router>
+        <RouterProvider router={appRouter} />
       </ToastProvider>
     </AuthProvider>
   )