feat(playback): show an "Up next" card in the last minutes of an episode

Phase 4 of the "Hybrid" player controls redesign. Series hosts hand the
episode after the playing one to the shared controls (`upNext` on
PortalInlinePlayerComponent → WebPlayerViewComponent → the four engine
hosts → app-player-controls). ControlsUpNext shows the glass card once at
most eight minutes remain on a seekable episode with a next-episode
capable engine; it yields to the settings panel and to hidden controls. A
click emits nextEpisodeRequested, so the switch keeps fullscreen like the
transport button. Compact docks get a smaller card.

Also dedupes the web-player-view spec's local engine stubs in favour of
the shared stub file its sibling specs already use.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.changes/playback-up-next-card.md

@@ -0,0 +1,9 @@
+---
+type: feature
+area: playback
+highlight: Up next in the player
+---
+
+In the last minutes of a series episode the player shows an "Up next" card
+with the following episode's still, title and time remaining. One click
+plays it, and the switch keeps you in fullscreen.