fix(settings): show the locked-groups row on the M3U rail and roll the relock timeout back to the recovered value

- The M3U groups rail now renders the same "N locked · Enter PIN to show"
  row as the portal category rail, so locked groups no longer vanish
  without an in-context unlock.
- A failed relock-timeout write rolls back to the value read after the
  settings retry, not to the pre-retry default.

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

docs/architecture/parental-lock.md

@@ -223,6 +223,10 @@ on either side.
   whenever the store selection is cleared: its template mounts the player
   only with a selection, and the held stream must not resurface with the
   next one.
+- **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.
 - **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