fix(stalker): keep live playback when switching the ITV/radio category (#1517)

Switching the Live TV or radio category in the shell context panel tore
down the Stalker player: `onStalkerCategoryClicked` cleared the selected
item for every section, and the live layout gates its player on
`selectedItem`. Xtream live (#936) and M3U groups already keep the channel
playing across a category/group switch.

- Context panel: return before `clearSelectedItem()` for `itv`/`radio`;
  VOD/series clicks still drop the open detail before navigating.
- Live layout: the category-change reset effect no longer wipes the
  playing channel's short-EPG fallback or cancels a fallback load in
  flight; only a section change (itv <-> radio) does that now.
- Regression coverage in the context panel spec, the live layout spec and
  a new web E2E scenario; docs and a `.changes/` note added.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

.changes/stalker-keep-live-playback-on-category-switch.md

@@ -0,0 +1,9 @@
+---
+type: fix
+area: stalker
+---
+
+In Stalker portals, switching the Live TV or radio category in the sidebar no
+longer stops the channel that is playing. The category only changes which
+channels are listed, matching how Xtream portals and M3U playlists already
+behave.