Header: move Import link into the admin dropdown menu

The /sprints/import link no longer sits inline in the header next to
"New sprint"; it now lives at the top of the admin section of the
hamburger menu (above Workers / Users / Audit log / Settings).
"New sprint" stays in the header as the one quick-action admins reach
for from any page. Pure layout.twig edit — route, ImportController,
CSRF, and admin gating are all unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

views/layout.twig

@@ -22,7 +22,6 @@
                     <a href="/" class="text-slate-600 hover:text-slate-900 hover:underline dark:text-slate-300 dark:hover:text-slate-100">Sprints</a>
                     {% if currentUser.isAdmin %}
                         <a href="/sprints/new" class="text-slate-600 hover:text-slate-900 hover:underline dark:text-slate-300 dark:hover:text-slate-100">New sprint</a>
-                        <a href="/sprints/import" class="text-slate-600 hover:text-slate-900 hover:underline dark:text-slate-300 dark:hover:text-slate-100">Import</a>
                     {% endif %}
                     <span class="text-slate-400 dark:text-slate-600">·</span>
                     <span class="text-slate-600 dark:text-slate-300">
@@ -55,6 +54,8 @@
                              x-cloak
                              class="absolute right-0 mt-2 min-w-[12rem] rounded-md border border-slate-200 bg-white shadow-lg py-1 z-10 dark:bg-slate-800 dark:border-slate-700">
                             {% if currentUser.isAdmin %}
+                                <a href="/sprints/import" role="menuitem"
+                                   class="block px-3 py-2 text-sm text-slate-700 hover:bg-slate-50 focus:outline-none focus:ring-2 focus:ring-slate-400 dark:text-slate-200 dark:hover:bg-slate-700">Import</a>
                                 <a href="/workers" role="menuitem"
                                    class="block px-3 py-2 text-sm text-slate-700 hover:bg-slate-50 focus:outline-none focus:ring-2 focus:ring-slate-400 dark:text-slate-200 dark:hover:bg-slate-700">Workers</a>
                                 <a href="/users" role="menuitem"