UI consistency (page shell 3/N): playlist-explorer adopts .page-shell card

Convert the playlist-explorer page from a flat padded container to the
.page-shell floating card. Drop its bespoke `padding: 24px 32px`; keep the
full-height flex layout (display:flex / column / min-height:100%) since the
explorer fills the viewport.

Visible change by design. Watch: the full-height min-height:100% inside a
margin:20px card may run slightly tall — to be confirmed visually.

webui/index.html

@@ -3632,7 +3632,7 @@
 
             <!-- Playlist Explorer Page -->
             <div class="page" id="playlist-explorer-page">
-                <div class="explorer-container">
+                <div class="page-shell explorer-container">
                     <!-- Header (compact) -->
                     <div class="dashboard-header" style="margin-bottom: 12px;">
                         <div class="header-text">

webui/static/style.css

@@ -54451,7 +54451,7 @@ tr.tag-diff-same {
    ================================================================================== */
 
 .explorer-container {
-    padding: 24px 32px;
+    /* card styling from .page-shell; keep full-height flex layout */
     display: flex;
     flex-direction: column;
     min-height: 100%;