Add admin badge to navbar avatar (#36790)

Replace the standalone site admin icon in the navbar with a
Discord-style shield badge on the user avatar.

<img width="278" height="73" alt="image"
src="https://github.com/user-attachments/assets/0b074006-30b9-43c6-8ef2-2120e32e139a"
/>


Fixes: https://github.com/go-gitea/gitea/issues/35904

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

templates/base/head_navbar_icons.tmpl

@@ -17,9 +17,4 @@
 			<span class="notification_count{{if not $notificationUnreadCount}} tw-hidden{{end}}">{{$notificationUnreadCount}}</span>
 		</div>
 	</a>
-	{{if $data.IsSiteAdmin}}
-	<a class="item {{$itemExtraClass}}" href="{{AppSubUrl}}/-/admin" data-tooltip-content="{{ctx.Locale.Tr "admin_panel"}}">
-		{{svg "octicon-server"}}
-	</a>
-	{{end}}
 {{end}}