UI consistency (page shell 5/N): settings adopts .page-shell card

Settings was the one flat page with no single wrapper — its .dashboard-header
and .settings-content sat as siblings directly under .page. Wrap both in a
single .page-shell div so the page becomes a floating card with the header
banner at the top, matching the dashboard structure. HTML-only change (no CSS:
.settings-content keeps its minor `0 4px` inner padding).

Library is intentionally NOT converted — its full-height artist grid + A-Z
jump rail overflow a margin:20px card, so it stays flat as a documented
exception (same category as search/discover/active-downloads).

webui/index.html

@@ -3735,6 +3735,7 @@
 
             <!-- Settings Page -->
             <div class="page" id="settings-page">
+                <div class="page-shell">
                 <div class="dashboard-header">
                     <div class="header-text">
                         <h2 class="header-title"><img src="/static/settings.png" class="page-header-icon" alt=""><span>Settings</span></h2>
@@ -6200,6 +6201,7 @@
                         <button class="save-button" id="save-settings">Save Settings</button>
                     </div>
                 </div>
+                </div>
             </div>
 
             <!-- Hydrabase Page (Dev Mode Only) -->