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/Impressum.vue

@@ -55,7 +55,7 @@ const legalNotes = [
       </div>
 
       <div class="hero-actions hoard-action-row">
-        <v-btn color="primary" prepend-icon="mdi-home" to="/">Zur Startseite</v-btn>
+        <v-btn color="primary" prepend-icon="mdi-home" to="/welcome">Zur Startseite</v-btn>
         <v-btn variant="outlined" prepend-icon="mdi-login" to="/login">Zum Login</v-btn>
       </div>
     </section>