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.

backend/app/routes/channels.py

@@ -230,6 +230,7 @@ def _channel_detail_dict(
     blocked: bool,
     stored: int,
     deep_requested: bool,
+    priority: int,
     tag_ids: list[int],
 ) -> dict:
     return {
@@ -255,6 +256,9 @@ def _channel_detail_dict(
         # Deep-backfill state, so the channel page can offer "get full history" when the catalog
         # holds only the recent-backfill/RSS subset (video_count is YouTube's raw total).
         "deep_requested": deep_requested,
+        # This user's per-subscription feed priority (0 when not subscribed) so the channel page can
+        # offer the same priority stepper as the manager. Editable only while subscribed.
+        "priority": priority,
         "backfill_done": channel.backfill_done,
         "from_explore": channel.from_explore,
         "details_synced": channel.details_synced_at is not None,
@@ -315,6 +319,7 @@ def channel_detail(
         blocked=blocked,
         stored=int(stored),
         deep_requested=bool(sub.deep_requested) if sub else False,
+        priority=int(sub.priority) if sub else 0,
         tag_ids=tag_ids,
     )