feat: add Profile link to admin nav

The admin nav had no way back to the account management area. Add a
Profile link to /manage/profile, mirroring the Admin link already present
in the manage nav.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/porchlight/templates/admin/base.html

@@ -4,6 +4,7 @@
 <nav class="admin-nav" aria-label="Administration">
     <span class="admin-badge">Admin</span>
     <a href="/admin/users" {% if active_page == "users" %}aria-current="page"{% endif %}>Users</a>
+    <a href="/manage/profile">Profile</a>
     <button class="nav-logout" hx-post="/logout">Log out</button>
 </nav>
 {% block admin_content %}{% endblock %}