Hide admin panel from navigation and protect admin page

Remove HALLINTA link from nav, add subtle footer gear icon link.
Hide admin UI content until successful login (display:none by default).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

admin.html

@@ -235,6 +235,7 @@
     </div>
   </div>
 
+  <div id="adminContent" style="display:none">
   <header class="admin-header">
     <div class="logo">
       <span class="logo-icon">🍳</span>
@@ -409,6 +410,7 @@
 
     </div>
   </main>
+  </div><!-- /adminContent -->
 
   <div class="toast" id="toast"></div>
 
@@ -479,6 +481,7 @@
       const data = await apiPost('admin_login', { password: pw });
       if (data.ok) {
         document.getElementById('loginOverlay').style.display = 'none';
+        document.getElementById('adminContent').style.display = '';
         await loadAdminData();
       } else {
         err.textContent = data.error || 'Kirjautuminen epäonnistui.';
@@ -947,6 +950,7 @@
       const data = await apiGet('admin_check');
       if (data.loggedIn) {
         document.getElementById('loginOverlay').style.display = 'none';
+        document.getElementById('adminContent').style.display = '';
         await loadAdminData();
       } else {
         document.getElementById('adminPwInput').focus();