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/lib/plexDetailUi.tsx

@@ -77,32 +77,25 @@ export function Filterable({
   );
 }
 
-// Paint the item's art as a faint FIXED backdrop on the page's <main> scroll container (HTPC-style),
-// so the glass panels float over it. Cleared on unmount / when disabled / when there's no art.
+// Paint the item's art as a faint FULL-VIEWPORT backdrop (HTPC-style) so the glass chrome floats over
+// it. A dedicated `position:fixed inset-0` layer at z-index -1 sits BEHIND the (transparent / glassy)
+// app content — so the art fills the whole page edge-to-edge, under the nav rail and floating header
+// too, instead of only the <main> box (which is inset by the rail, the header and the scrollbar
+// gutter). Removed on unmount / when disabled / when there's no art.
 export function useArtBackdrop(art: string | null | undefined, enabled: boolean) {
   useLayoutEffect(() => {
-    const main = document.querySelector("main");
-    if (!main) return;
-    const s = main.style;
-    const clear = () => {
-      s.backgroundImage = "";
-      s.backgroundSize = "";
-      s.backgroundPosition = "";
-      s.backgroundRepeat = "";
-      s.backgroundAttachment = "";
-    };
-    if (enabled && art) {
-      s.backgroundImage =
-        `linear-gradient(color-mix(in srgb, var(--bg) 60%, transparent), ` +
-        `color-mix(in srgb, var(--bg) 64%, transparent)), url("${art}")`;
-      s.backgroundSize = "cover";
-      s.backgroundPosition = "center 20%";
-      s.backgroundRepeat = "no-repeat";
-      s.backgroundAttachment = "fixed";
-    } else {
-      clear();
-    }
-    return clear;
+    if (!enabled || !art) return;
+    const el = document.createElement("div");
+    el.setAttribute("data-plex-art-backdrop", "");
+    el.style.cssText =
+      "position:fixed;inset:0;z-index:-1;pointer-events:none;" +
+      "background-size:cover;background-position:center 20%;background-repeat:no-repeat;";
+    // The faint --bg overlay keeps it subtle and text-safe (same tuning the old <main> paint used).
+    el.style.backgroundImage =
+      `linear-gradient(color-mix(in srgb, var(--bg) 60%, transparent), ` +
+      `color-mix(in srgb, var(--bg) 64%, transparent)), url("${art}")`;
+    document.body.appendChild(el);
+    return () => el.remove();
   }, [art, enabled]);
 }