Add change-password API and dynamic 404 redirect

Introduce ChangePasswordRequest DTO and a new ChangePassword endpoint in AuthController that validates input, changes the user's password via UserManager, updates the security stamp, signs out the user to invalidate sessions, and returns localized messages. Add a simple authorized AppUserController stub (GET /auth/user). Update the 404 view to resolve auth status via fetchCurrentUser, show a dynamic CTA/icon (Dashboard vs Home), auto-redirect after a short delay with proper timer cleanup, and adjust navigation behavior. Update codexInfo.md to document the 404 behavior change.

GUI/src/routes/404NotFound.vue

@@ -1,18 +1,74 @@
 <script setup lang="ts">
+import { computed, onBeforeUnmount, onMounted, ref } from 'vue'
 import { useRouter } from 'vue-router'
 
 import notFoundImage from '@/assets/images/404NotFound.png'
+import { fetchCurrentUser } from '@/services/authSession'
 
 const router = useRouter()
+const isAuthenticated = ref(false)
+let autoRedirectTimer: ReturnType<typeof setTimeout> | null = null
+
+const redirectRouteName = computed(() => (isAuthenticated.value ? 'Dashboard' : 'Home'))
+const redirectButtonLabel = computed(() =>
+  isAuthenticated.value ? 'Zum Dashboard' : 'Zur Startseite',
+)
+const redirectButtonIcon = computed(() =>
+  isAuthenticated.value ? 'mdi-view-dashboard-outline' : 'mdi-home-outline',
+)
+const autoRedirectTargetLabel = computed(() => (isAuthenticated.value ? 'Dashboard' : 'Startseite'))
+
+function clearAutoRedirectTimer() {
+  if (autoRedirectTimer === null) {
+    return
+  }
+
+  clearTimeout(autoRedirectTimer)
+  autoRedirectTimer = null
+}
+
+function scheduleAutoRedirect() {
+  clearAutoRedirectTimer()
+
+  autoRedirectTimer = setTimeout(() => {
+    void router.replace({ name: redirectRouteName.value })
+  }, 2000)
+}
+
+async function resolveRedirectTarget() {
+  try {
+    const currentUser = await fetchCurrentUser()
+    isAuthenticated.value = currentUser !== null
+  } catch {
+    isAuthenticated.value = false
+  }
+
+  scheduleAutoRedirect()
+}
+
+function navigateToPrimaryTarget() {
+  clearAutoRedirectTimer()
+  void router.replace({ name: redirectRouteName.value })
+}
 
 function navigateBack() {
+  clearAutoRedirectTimer()
+
   if (window.history.length > 1) {
     router.back()
     return
   }
 
-  router.push({ name: 'Dashboard' })
+  void router.push({ name: redirectRouteName.value })
 }
+
+onMounted(() => {
+  void resolveRedirectTarget()
+})
+
+onBeforeUnmount(() => {
+  clearAutoRedirectTimer()
+})
 </script>
 
 <template>
@@ -29,12 +85,17 @@ function navigateBack() {
         <h1>Seite nicht gefunden</h1>
         <p class="not-found-text">
           Der Link ist ungültig oder die Seite wurde verschoben. Du kannst direkt zur
-          Dashboard-Seite zurück oder die vorherige Ansicht öffnen.
+          {{ autoRedirectTargetLabel }} weitergehen oder die vorherige Ansicht öffnen.
+          Du wirst automatisch dorthin weitergeleitet.
         </p>
 
         <div class="not-found-actions hoard-action-row">
-          <v-btn color="primary" prepend-icon="mdi-view-dashboard-outline" :to="{ name: 'Dashboard' }">
-            Zum Dashboard
+          <v-btn
+            color="primary"
+            :prepend-icon="redirectButtonIcon"
+            @click="navigateToPrimaryTarget"
+          >
+            {{ redirectButtonLabel }}
           </v-btn>
           <v-btn variant="outlined" prepend-icon="mdi-arrow-left" @click="navigateBack">Zurück</v-btn>
         </div>