feat(ui): E31 S4c — the channel header folds, the Plex pages fill their hero, the player gets a side panel

- Channel page: the banner and identity block fold away once the grid scrolls
  (ui/scrollFold: folds past 32px only if the page can stay scrolled, unfolds
  at the very top) and the name rides in the tab row. One Back button in both
  states keeps its focus; folded at a tablet width the actions wrap instead of
  pushing the page sideways.
- Plex movie and show pages: the TMDB facts over the key crew are the hero's
  second column (plexDetailAside), the keywords sit under the text; the show
  page shows them for the first time, with the same three toggles. A
  collection's hero is sized to its text.
- YouTube player, 1280px and wider: the title, channel, actions, description
  and queue sit in a side panel beside the video, which takes the window's
  height (1438x809 at 1920x911, was ~1008 wide). The wheel scrolls that panel
  while it can and turns the volume elsewhere.
- DESIGN.md decision log: the keywords' place under the text.

frontend/src/components/PlexCollectionView.tsx

@@ -286,6 +286,9 @@ export default function PlexCollectionView({
             posterDisabled={!canPlay || queueBusy}
             showPlayOverlay={canPlay}
             onPosterClick={() => canPlay && !queueBusy && playAll(false)}
+            // Sized to the text beside it (E31, the owner's pick): a title, one meta line and a short
+            // description sat beside a show-sized poster, two thirds of the hero empty under them.
+            posterSizeClass="w-20 sm:w-24"
             customizeMenu={null}
           >
             {renaming ? (