feat(ux): card unification, slim icon nav, filter/plex fixes (7-item batch)

1A ChannelPage "Full history" button no longer wraps (whitespace-nowrap).
1B Extract the manager's press-and-hold priority stepper into a shared
   PriorityStepper and add it to the channel detail header (subscribed, non-demo);
   backend channel_detail now returns the per-subscription `priority`.
2  Unify YouTube + Plex media cards onto one look/behaviour: drop the whole-card
   hover LIFT (VideoCard) — which also fixes the clipped-top bug — and instead the
   inner thumbnail/poster ZOOMS on hover (both); give the flat Plex posters the same
   glass-card 3D depth + accent hover border as the feed cards (shadow-md is a
   transparent no-op here, so it's glass-card that provides the depth); clamp Plex
   card titles to a fixed 2-line height so 1- vs 2-line titles align.
3  Nav rail is now always a slim icon-only column, flush to the left edge; each icon
   reveals a floating name label on hover (Tooltip gains `always` + `openDelay`, and
   a right side). Removed the pin/unpin toggle, the expand-on-hover mode and the
   navCollapsed pref plumbing.
4  Scope the side-panel peek-open HOVER to the tab handle + the card (not the whole
   full-height slot next to the rail), so approaching the nav no longer pops the
   filter open; focus stays container-wide. useHoverFocusWithin gains a hover-out
   grace period + split hover/focus handler groups.
5  Plex art backdrop fills the whole viewport (a fixed inset-0 layer behind the
   transparent app) instead of only the inset <main> box.
6  Plex season binge continues across seasons: the season view's play queue now
   spans the whole show (allKeys), so a season's last episode rolls into the next
   season's first (seasonKeys stays for "add season to playlist").
7  When a Plex video/episode ends with nothing queued next, the player returns to
   the previous page (onEnded → onClose) instead of stranding a black screen.

frontend/src/components/PlexPlayer.tsx

@@ -767,10 +767,13 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
       if (absRef.current <= 0) return;
       api.plexSetState(id, "watched").catch(() => {});
       if (nextId) go(nextId);
+      // Nothing queued next → dismiss the player back to the previous page (same as Stop/Escape),
+      // instead of stranding a black, ended <video> on screen.
+      else onClose();
     };
     video.addEventListener("ended", onEnded);
     return () => video.removeEventListener("ended", onEnded);
-  }, [id, nextId, go]);
+  }, [id, nextId, go, onClose]);
 
   // Reveal the controls and re-arm the auto-hide timer (on mouse move OR any key).
   const wake = useCallback(() => {