UI consistency (page shell 6/N): sync page adopts .page-shell card

Standardize the sync page's outer spacing to match the other pages. Like
settings, its .sync-header and .sync-content-area were siblings directly under
.page (no wrapper) — wrap both in a single .page-shell div so it becomes the
floating card with consistent margin/padding. HTML-only change.

Watch: .sync-content-area uses height:95% (grid) — fine against an auto-height
card, but to be confirmed visually (library's full-height grid was the one
that didn't fit a card).

webui/index.html

@@ -976,6 +976,7 @@
 
             <!-- Main container for the Sync page -->
             <div class="page" id="sync-page">
+                <div class="page-shell">
                 <!-- Header -->
                 <div class="sync-header">
                     <div class="sync-header-row">
@@ -2059,6 +2060,7 @@
                         </div>
                     </div>
                 </div>
+                </div>
             </div>
 
             <!-- Search Page -->