Replace admin Back button with UserMenu

Remove the Back button and its sessionStorage return-URL stash
logic. Add UserMenu to the right side of the admin topbar,
providing consistent navigation to all surfaces and eliminating
the infinite-loop problem (Back → / → admin → Back → ...).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/js/sw/surfaces/admin/index.js

@@ -15,6 +15,7 @@ const { render } = preact;
 
 import { ToastContainer } from '../../primitives/toast.js';
 import { DialogStack } from '../../shell/dialog-stack.js';
+import { UserMenu } from '../../shell/user-menu.js';
 
 // ── Category → Section mapping ──────────────
 const ADMIN_SECTIONS = {
@@ -121,26 +122,6 @@ function AdminSurface() {
         }
     }, [section]);
 
-    // Back button with return URL stash
-    useEffect(() => {
-        const RETURN_KEY = 'sb_admin_return';
-        if (!sessionStorage.getItem(RETURN_KEY)) {
-            const ref = document.referrer;
-            if (ref && ref.startsWith(location.origin) && !ref.includes('/admin')) {
-                sessionStorage.setItem(RETURN_KEY, ref);
-            } else {
-                sessionStorage.setItem(RETURN_KEY, location.origin + BASE + '/');
-            }
-        }
-    }, []);
-
-    const backClick = useCallback((e) => {
-        e.preventDefault();
-        const RETURN_KEY = 'sb_admin_return';
-        const returnURL = sessionStorage.getItem(RETURN_KEY);
-        sessionStorage.removeItem(RETURN_KEY);
-        location.href = returnURL || BASE + '/';
-    }, []);
 
     const navClick = useCallback((e) => {
         e.preventDefault();
@@ -155,15 +136,6 @@ function AdminSurface() {
         <div class="surface-admin">
             ${/* Top Bar */``}
             <div class="admin-topbar">
-                <a href="${BASE}/" class="admin-topbar-back" onClick=${backClick}>
-                    <svg width="15" height="15" viewBox="0 0 24 24" fill="none" stroke="currentColor"
-                         stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
-                        <line x1="19" y1="12" x2="5" y2="12"/>
-                        <polyline points="12 19 5 12 12 5"/>
-                    </svg>
-                    Back
-                </a>
-                <div class="admin-topbar-sep"></div>
                 <span class="admin-topbar-title">
                     <img src="${BASE}/favicon.svg" alt="" width="18" height="18" style="vertical-align:-3px;" />
                     Administration
@@ -179,6 +151,10 @@ function AdminSurface() {
                         </a>
                     `)}
                 </div>
+
+                <div style="margin-left:auto;">
+                    <${UserMenu} placement="down-right" />
+                </div>
             </div>
 
             <div class="admin-body">