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/_partials/page_placeholder.html.twig

@@ -0,0 +1,13 @@
+{# Temporary empty state for pages whose data model isn't built yet. #}
+<div class="d-flex justify-content-between align-items-center mb-3">
+    <h1 class="h3 mb-0">{{ title }}</h1>
+</div>
+
+<div class="card shadow-sm">
+    <div class="card-body text-center py-5">
+        <div class="display-6 mb-2">{{ icon }}</div>
+        <p class="text-body-secondary mb-0">
+            {{ blurb }}
+        </p>
+    </div>
+</div>