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>

templates/storyline/index.html.twig

@@ -0,0 +1,11 @@
+{% extends 'base.html.twig' %}
+
+{% block title %}Storyline · dmtools{% endblock %}
+
+{% block body %}
+    {{ include('_partials/page_placeholder.html.twig', {
+        title: 'Storyline',
+        icon: '📜',
+        blurb: 'The campaign storyline will live here. Shape still to be decided — session log, plot threads, or a wiki-style document.',
+    }) }}
+{% endblock %}