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/VideoCard.tsx

@@ -239,7 +239,7 @@ function Thumb({
           decoding="async"
           width={1280}
           height={720}
-          className="w-full h-full object-cover"
+          className="w-full h-full object-cover transition-transform duration-200 group-hover:scale-105"
         />
       ) : (
         <div className="w-full h-full" />
@@ -598,7 +598,7 @@ function VideoCard({
       data-testid="video-card"
       data-video-id={video.id}
       className={clsx(
-        "cv-card group glass-card glass-hover rounded-2xl transition-all duration-150 hover:-translate-y-1",
+        "cv-card group glass-card glass-hover rounded-2xl transition-all duration-150",
         "flex flex-col",
         spec.dense ? "p-2" : "p-2.5",
         watched && "opacity-55",