Add nav menu with Characters, NPC's and Storyline pages

Navbar (collapsible) with active-state highlighting driven by a nav list
in base.html.twig. Three routed, auth-gated controllers each rendering a
shared placeholder partial - data models still to be designed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/Controller/StorylineController.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace App\Controller;
+
+use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\Routing\Attribute\Route;
+use Symfony\Component\Security\Http\Attribute\IsGranted;
+
+#[IsGranted('IS_AUTHENTICATED_FULLY')]
+class StorylineController extends AbstractController
+{
+    #[Route('/storyline', name: 'app_storyline')]
+    public function index(): Response
+    {
+        return $this->render('storyline/index.html.twig');
+    }
+}