fix(settings): keep the M3U groups view when every group is locked

With every group locked the filtered channel list is empty, so the
container showed its generic empty state and the groups rail's
"N locked · Enter PIN to show" row never appeared.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture/parental-lock.md

@@ -229,7 +229,10 @@ on either side.
 - **M3U groups rail:** `ChannelListContainerComponent` derives
   `withheldGroupCount` (groups of the playlist the active lock withholds)
   and the groups rail renders the same "N locked · Enter PIN to show" row
-  as the portal category rail, so locked groups do not simply vanish.
+  as the portal category rail, so locked groups do not simply vanish —
+  also when EVERY group is locked: the container keeps the groups view
+  (`showChannelViews`) and the groups view keeps its rail instead of the
+  generic empty state.
 - **Xtream (PWA):** `PwaXtreamDataSource` drops withheld categories,
   streams and search hits at read time; the same reloads apply.
 - **Warm-cache detection (Electron):** the filtered category/content reads