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/ShareDialog.tsx

@@ -56,7 +56,7 @@ function UserShare({ job }: { job: DownloadJob }) {
           className={inputCls}
         />
         {open && filtered.length > 0 && (
-          <div className="absolute z-10 mt-1 w-full rounded-lg border border-border bg-card shadow-xl overflow-hidden">
+          <div className="absolute z-10 mt-1 w-full rounded-lg glass-menu shadow-xl overflow-hidden">
             {filtered.map((u) => (
               <button
                 key={u.id}
@@ -110,7 +110,7 @@ function LinkRow({ link, onChanged }: { link: ShareLink; onChanged: () => void }
   if (link.expires_at) badges.push(t("downloads.share.expiresOn", { date: new Date(link.expires_at).toLocaleDateString() }));
 
   return (
-    <div className="rounded-lg bg-card/40 p-2.5 space-y-2">
+    <div className="rounded-lg glass-card p-2.5 space-y-2">
       <div className="flex items-center gap-2">
         <Link2 className="w-4 h-4 text-muted shrink-0" />
         <input readOnly value={fullUrl} className="flex-1 min-w-0 bg-transparent text-xs text-muted truncate outline-none" />