feat(glass): glassify modal inner panels, dropdowns & list rows (Phase 2)

Medium container surfaces -> the glass system so they refract the backdrop
consistently: Playlists row (bg-card->glass-card), DownloadCenter user-picker +
ShareDialog share-picker dropdowns (bg-card->glass-menu), DownloadCenter stat
cards / download rows / usage block, ShareDialog share-link block, VideoEditor
selection bar, ProfileEditor profile rows (bg-card/40|bg-surface/60->glass-card).
Tight admin rows and tiny inputs left solid on purpose.

frontend/src/components/Playlists.tsx

@@ -123,7 +123,7 @@ function Row({
     <div
       ref={setNodeRef}
       style={style}
-      className="flex items-center gap-2.5 p-2 rounded-lg border border-border bg-card"
+      className="flex items-center gap-2.5 p-2 rounded-lg glass-card glass-hover"
     >
       {readOnly ? (
         <span className="w-4" />