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>

index.html

@@ -22,7 +22,6 @@
         <a href="#posts" data-i18n="nav_posts"></a>
         <a href="#about" data-i18n="nav_about"></a>
         <a href="#contact" data-i18n="nav_contact"></a>
-        <a href="admin.html" data-i18n="nav_admin"></a>
       </nav>
     </div>
   </header>
@@ -76,6 +75,7 @@
   <!-- FOOTER -->
   <footer>
     <p>🍳 tykkää.fi &mdash; <span data-i18n="footer"></span> &copy; 2026</p>
+    <a href="admin.html" style="color:#c4b09a;font-size:0.7rem;text-decoration:none;opacity:0.4">⚙</a>
   </footer>
 
   <!-- POST MODAL -->