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/VideoEditor.tsx

@@ -443,7 +443,7 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
 
         {/* selected-segment editor */}
         {sel && (
-          <div className="flex flex-wrap items-center gap-2 text-sm rounded-lg bg-card/40 px-3 py-2">
+          <div className="flex flex-wrap items-center gap-2 text-sm rounded-lg glass-card px-3 py-2">
             <span className="font-medium">{t("editor.segment", { n: selIdx + 1 })}</span>
             <button
               onClick={() => toggleKeep(sel.id)}