Move user management button to header

Users button now appears next to the logged-in user info
in the top right header instead of as a tab. Only visible
to admin users.

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

index.html

@@ -64,6 +64,7 @@
             </div>
             <div class="header-right">
                 <span id="user-info" class="user-info"></span>
+                <button id="btn-users" class="btn-secondary" style="display:none">Käyttäjät</button>
                 <button id="btn-add" class="btn-primary">+ Lisää asiakas</button>
                 <button id="btn-logout" class="btn-secondary">Kirjaudu ulos</button>
             </div>
@@ -75,7 +76,6 @@
             <button class="tab" data-tab="leads">Liidit</button>
             <button class="tab" data-tab="archive">Arkisto</button>
             <button class="tab" data-tab="changelog">Muutosloki</button>
-            <button class="tab" data-tab="users" id="tab-users" style="display:none">Käyttäjät</button>
             <button class="tab" data-tab="settings" id="tab-settings" style="display:none">API</button>
         </div>