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/DownloadCenter.tsx

@@ -189,7 +189,7 @@ function DownloadRow({
   const { t } = useTranslation();
   const running = job.status === "running";
   return (
-    <div className="flex gap-3 p-2.5 rounded-xl bg-card/40 hover:bg-card transition">
+    <div className="flex gap-3 p-2.5 rounded-xl glass-card glass-hover transition">
       <Thumb job={job} />
       <div className="min-w-0 flex-1">
         <div className="font-medium leading-snug line-clamp-1 flex items-center gap-1.5">
@@ -338,7 +338,7 @@ function UsageBar() {
   if (!u) return null;
   const pct = u.unlimited || !u.max_bytes ? 0 : Math.min(100, (u.footprint_bytes / u.max_bytes) * 100);
   return (
-    <div className="rounded-xl bg-card/40 p-3 mb-4">
+    <div className="rounded-xl glass-card p-3 mb-4">
       <div className="flex justify-between text-sm mb-1.5">
         <span className="font-medium">{t("downloads.usage.title")}</span>
         <span className="text-muted">
@@ -446,7 +446,7 @@ function QuotaUserPicker({ onPick }: { onPick: (u: { id: number; email: string }
         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}
@@ -482,7 +482,7 @@ function AdminSystem() {
             [t("downloads.admin.totalSize"), formatBytes(s.total_bytes)],
             [t("downloads.admin.cap"), s.total_cap_bytes ? formatBytes(s.total_cap_bytes) : t("downloads.admin.noCap")],
           ].map(([label, val]) => (
-            <div key={label} className="rounded-xl bg-card/40 p-3">
+            <div key={label} className="rounded-xl glass-card p-3">
               <div className="text-xs text-muted">{label}</div>
               <div className="text-lg font-semibold mt-0.5">{val}</div>
             </div>