fix(playlists): hide '#' and Channel by the table's width, not the window's (E28)

Beside the playlist rail an 800 px window left the table 432 px: the fixed
columns took all of it and the title column was 0 px wide, its thumbnail
drawn over the channel. The table is an @container now and the columns hide
under 42rem of its own width (narrowHidden, was phoneHidden).

frontend/src/components/playlistColumns.tsx

@@ -38,7 +38,7 @@ export function playlistColumns({
       nowrap: true,
       hideInCard: true,
       // The position number is the first thing a phone row can spare; the order still shows.
-      phoneHidden: true,
+      narrowHidden: true,
       // Sorting by "#" ascending is the stored order itself — that is what makes it a truthful
       // default for the header arrow, and a one-click way back from any other sort.
       sortable: true,
@@ -89,7 +89,7 @@ export function playlistColumns({
       key: "channel",
       header: t("playlists.colChannel"),
       width: "12rem",
-      phoneHidden: true,
+      narrowHidden: true,
       nowrap: true,
       sortable: true,
       sortValue: (v) => v.channel_title ?? "",