Add Dashboard nav link for authenticated users

- Add conditional "Dashboard" link in navbar that shows when a user is logged in
- Tests verify the link is hidden for unauthenticated users and visible when authenticated
- Investigation reports for Q1 (drafts/scheduled), Q2 (RSS), Q3 (email subs)

app/lib/firehose_web/components/layouts/app.html.heex

@@ -23,6 +23,9 @@
       <li>
         <a href="/contact" class="btn btn-ghost btn-sm">Contact</a>
       </li>
+      <li :if={@current_scope && @current_scope.user}>
+        <a href="/editor/dashboard" class="btn btn-ghost btn-sm">Dashboard</a>
+      </li>
       <li>
         <.theme_toggle />
       </li>