feat(dashboard): portal EPG on the live rails, loaded lazily per visible card (#1638)

Xtream and Stalker live cards on the dashboard carried no XMLTV key, so the
rails never asked anything for them and showed only the LIVE chip. Their
programme now comes from the portal, one card at a time and only once the card
is on screen.

- `lib-dashboard-rail` reports the cards inside its viewport through an
  IntersectionObserver rooted at the track; `DashboardLiveEpgPresenter` unions
  them with the pinned hero row and hands the set to
  `DashboardPortalLiveEpgPresenter`.
- `DashboardPortalLiveEpgService` runs the bounded queue — two requests in
  flight, 200 ms apart, one card each — through
  `StreamResolverService.loadEpgForItems`, publishing every answer the moment
  it lands, so the page never waits and a slow portal delays no other card. A
  card scrolled past before its turn is never requested.
- A programme is trusted for 60 s, an empty answer for 30 s (the resolver
  reports a dead portal and a guide-less channel identically), and a
  completion captures both the display offset and the EPG source revision,
  requeueing itself when either moved.
- The presenter hands its wanted set back on destroy. Desktop only: the shared
  collection resolver is gated on the local XMLTV bridge.
- A shimmer placeholder shows only before a card's first portal answer; M3U
  cards keep the batched XMLTV lookup.

.changes/dashboard-portal-live-epg.md

@@ -0,0 +1,9 @@
+---
+type: feature
+area: dashboard
+---
+
+On the desktop app, favourite and recently watched Xtream and Stalker channels
+on the dashboard now show what is on air, with the programme's time and
+progress. Each card asks its portal only once it scrolls into view and fills in
+on its own, so a slow portal never holds up the page or the other cards.