feat(admin): add box preview and password bypass for administrators

Introduce an `AdminViewBox` handler and route that allows administrators
to view any box directly. If the box is password-protected, the handler
bypasses the protection by setting an unlock cookie with an unlock token
and logs the bypass event.

Additionally, add CSS and JS foundations for a file context menu and
preview actions in the file browser UI.

backend/templates/pages/admin.html

@@ -77,7 +77,7 @@
                   {{if .Protected}}<span class="badge">protected</span>{{end}}
                 </td>
                 <td class="table-actions">
-                  <a class="button button-outline" href="/d/{{.ID}}">View</a>
+                  <a class="button button-outline" href="/admin/boxes/{{.ID}}/view">View</a>
                   <form action="/admin/boxes/{{.ID}}/delete" method="post">
                     <button class="button button-danger" type="submit">Delete</button>
                   </form>