Populate admin dropdown menu with all admin page routes

[Copilot]

The Admin dropdown in the header navbar was rendering with no items — visible toggle, empty menu.

Changes

• app.html.heex — Added route links to the Admin dropdown, grouped with dividers:
  • Members, Cohorts, Projects
  • Join Requests
  • GitHub Whitelist, GitHub Blacklist

<ul class="dropdown-menu">
  <li><a href={~p"/admin/members"} class="dropdown-item">Members</a></li>
  <li><a href={~p"/admin/cohorts"} class="dropdown-item">Cohorts</a></li>
  <li><a href={~p"/admin/projects"} class="dropdown-item">Projects</a></li>
  <li><hr class="dropdown-divider" /></li>
  <li><a href={~p"/admin/join_requests"} class="dropdown-item">Join Requests</a></li>
  <li><hr class="dropdown-divider" /></li>
  <li><a href={~p"/admin/allowed_handles"} class="dropdown-item">GitHub Whitelist</a></li>
  <li><a href={~p"/admin/blacklisted_handles"} class="dropdown-item">GitHub Blacklist</a></li>
</ul>

The dropdown remains gated behind @current_member.is_admin, so it is invisible to non-admin users.