panel: place drill-down back links under the heading

The user form had Back to users at the bottom of the card; all drill-down
pages now use a shared back_link template with a structural test.

Co-Authored-By: Claude <Composer 2.5> <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/view/templates/user_form.html

@@ -1,6 +1,8 @@
 {{define "content"}}
 <h1>{{if .IsEdit}}Edit user{{else}}Create user{{end}}</h1>
 
+{{template "back_link" (back "/users" "Back to users")}}
+
 <div class="card narrow">
   {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
   <form method="post" action="{{if .IsEdit}}/users/{{.UserID}}{{else}}/users/new{{end}}">
@@ -33,6 +35,5 @@
     <button type="submit" name="action" value="delete" class="danger">Delete user</button>
     {{end}}
   </form>
-  <p class="muted"><a href="/users">Back to users</a></p>
 </div>
 {{end}}