Add frontend auth, dashboard & router guards

Introduce a complete frontend auth flow and protected dashboard.

- Add auth session module (GUI/src/services/authSession.ts) with fetchCurrentUser, login, logout, caching and structured errors.
- Add Dashboard page (GUI/src/routes/dashboard/Dashboard.vue) and a protected Dashboard route (meta.requiresAuth) at '/'.
- Move public landing page to /welcome and mark it Visibility.Unauthenticated; update 404 and Impressum links.
- Implement router guard (GUI/src/router/index.ts) to redirect unauthenticated users to Login and prevent logged-in users from accessing guest-only pages.
- Update routes layout (GUI/src/plugins/routesLayout.ts) to include authenticated/unauthenticated visibility and dashboard entry.
- Update Layout.vue to track current user, show username/menu, conditionally render sidebar items, add logout flow and error snackbar, and insert visual divider before auth-only items.
- Convert Login.vue into a working login form with loading state, error handling and redirect after success.
- Update codexInfo.md to document the new auth features and related UI/route changes.

GUI/src/routes/authentication/Login.vue

@@ -1,7 +1,56 @@
 <script setup lang="ts">
-import { ref } from 'vue'
+import { computed, ref } from 'vue'
+import { useRoute, useRouter } from 'vue-router'
+import { AuthRequestError, login } from '@/services/authSession'
 
 const showPassword = ref(false)
+const userName = ref('')
+const password = ref('')
+const isSubmitting = ref(false)
+const errorMessage = ref('')
+
+const route = useRoute()
+const router = useRouter()
+
+const submitDisabled = computed(
+  () => isSubmitting.value || userName.value.trim().length === 0 || password.value.length === 0,
+)
+
+const redirectPath = computed(() => {
+  const redirectQuery = route.query.redirect
+  if (typeof redirectQuery === 'string' && redirectQuery.startsWith('/')) {
+    return redirectQuery
+  }
+
+  return '/'
+})
+
+async function handleSubmit() {
+  if (submitDisabled.value) {
+    errorMessage.value = 'Bitte Benutzername und Passwort eingeben.'
+    return
+  }
+
+  isSubmitting.value = true
+  errorMessage.value = ''
+
+  try {
+    await login({
+      userName: userName.value.trim(),
+      password: password.value,
+    })
+
+    await router.replace(redirectPath.value)
+  } catch (error) {
+    if (error instanceof AuthRequestError) {
+      errorMessage.value = error.message
+    } else {
+      errorMessage.value = 'Anmeldung fehlgeschlagen. Bitte versuche es erneut.'
+    }
+  } finally {
+    isSubmitting.value = false
+  }
+}
 </script>
 
 <template>
@@ -30,22 +79,35 @@ const showPassword = ref(false)
         </ul>
       </aside>
 
-      <v-form class="login-form hoard-panel" @submit.prevent>
+      <v-form class="login-form hoard-panel" @submit.prevent="handleSubmit">
         <div class="form-head">
           <h2>Login</h2>
           <p>Melde dich mit deinem bestehenden Konto an.</p>
         </div>
 
+        <v-alert
+          v-if="errorMessage"
+          type="error"
+          variant="tonal"
+          density="comfortable"
+          border="start"
+        >
+          {{ errorMessage }}
+        </v-alert>
+
         <v-text-field
-          label="E-Mail"
-          type="email"
+          v-model="userName"
+          label="Benutzername"
+          type="text"
           variant="outlined"
-          prepend-inner-icon="mdi-email-outline"
-          autocomplete="email"
+          prepend-inner-icon="mdi-account-outline"
+          autocomplete="username"
           required
+          :disabled="isSubmitting"
         />
 
         <v-text-field
+          v-model="password"
           label="Passwort"
           :type="showPassword ? 'text' : 'password'"
           variant="outlined"
@@ -53,17 +115,27 @@ const showPassword = ref(false)
           :append-inner-icon="showPassword ? 'mdi-eye-off-outline' : 'mdi-eye-outline'"
           autocomplete="current-password"
           required
+          :disabled="isSubmitting"
           @click:append-inner="showPassword = !showPassword"
         />
 
         <div class="form-meta">
-          <v-checkbox hide-details color="primary" density="compact" label="Angemeldet bleiben" />
-          <v-btn variant="text" size="small">Passwort vergessen?</v-btn>
+          <p>Anmeldung erfolgt per sicherem Session-Cookie.</p>
         </div>
 
-        <v-btn type="submit" color="primary" block size="large" prepend-icon="mdi-login">Anmelden</v-btn>
+        <v-btn
+          type="submit"
+          color="primary"
+          block
+          size="large"
+          prepend-icon="mdi-login"
+          :loading="isSubmitting"
+          :disabled="submitDisabled"
+        >
+          Anmelden
+        </v-btn>
 
-        <v-btn variant="outlined" block to="/" prepend-icon="mdi-home">Zur Startseite</v-btn>
+        <v-btn variant="outlined" block to="/welcome" prepend-icon="mdi-home">Zur Startseite</v-btn>
       </v-form>
     </section>
   </v-container>
@@ -132,8 +204,13 @@ h1 {
 .form-meta {
   display: flex;
   align-items: center;
-  justify-content: space-between;
-  gap: var(--space-3);
+  gap: var(--space-2);
+}
+
+.form-meta p {
+  margin: 0;
+  color: var(--color-text-muted);
+  font-size: var(--font-size-sm);
 }
 
 @media (width <= 960px) {
@@ -179,11 +256,6 @@ h1 {
     gap: var(--space-2);
   }
 
-  :deep(.form-meta .v-btn) {
-    width: 100%;
-    min-height: 44px;
-  }
-
   :deep(.login-form .v-btn) {
     min-height: 44px;
   }