fix: nav slash, back button context, home page max-width

- Past Trips nav link: add missing / (base_url_absolute has no trailing slash)
- Entry back link: history.back() with journal fallback, label → "← Back"
- Home page: max-width 1400px instead of none — narrows layout on wide screens

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RB86BaJBG3eGiMdfhmHRrQ

themes/intotheeast/templates/partials/base.html.twig

@@ -18,7 +18,7 @@
         {% block nav %}
         <nav class="site-nav" aria-label="Main navigation">
             <a href="{{ base_url_absolute }}"{% if page.template == 'home' %} aria-current="page"{% endif %}>Home</a>
-            <a href="{{ base_url_absolute }}trips"{% if page.template == 'trips' %} aria-current="page"{% endif %}>Past Trips</a>
+            <a href="{{ base_url_absolute }}/trips"{% if page.template == 'trips' %} aria-current="page"{% endif %}>Past Trips</a>
         </nav>
         {% endblock %}
     </header>