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/App.tsx

@@ -92,10 +92,6 @@ export default function App() {
   // Collapse state for the two full-height panels (left nav + filter sidebar). Persisted to the
   // user's preferences so it follows the account across devices; a localStorage cache seeds the
   // initial render synchronously so nothing flashes open before the server prefs arrive.
-  // Default unpinned (slim + hover-expand overlay); accounts that explicitly pin keep their choice.
-  const [navCollapsed, setNavCollapsedState] = useState<boolean>(() =>
-    Boolean(readAccount(LS.navCollapsed, true)),
-  );
   // Default unpinned (tab + hover-expand overlay); accounts that explicitly pin keep their choice.
   const [filterCollapsed, setFilterCollapsedState] = useState<boolean>(() =>
     Boolean(readAccount(LS.filterCollapsed, true)),
@@ -104,11 +100,6 @@ export default function App() {
   const [playlistsCollapsed, setPlaylistsCollapsedState] = useState<boolean>(() =>
     Boolean(readAccount(LS.playlistsCollapsed, true)),
   );
-  function setNavCollapsed(next: boolean) {
-    setNavCollapsedState(next);
-    writeAccount(LS.navCollapsed, next);
-    api.savePrefs({ navCollapsed: next }).catch(() => {});
-  }
   function setFilterCollapsed(next: boolean) {
     setFilterCollapsedState(next);
     writeAccount(LS.filterCollapsed, next);
@@ -236,10 +227,6 @@ export default function App() {
         saveLayoutLocal(p, l);
       }
     });
-    if (typeof prefs.navCollapsed === "boolean") {
-      setNavCollapsedState(prefs.navCollapsed);
-      writeAccount(LS.navCollapsed, prefs.navCollapsed);
-    }
     if (typeof prefs.filterCollapsed === "boolean") {
       setFilterCollapsedState(prefs.filterCollapsed);
       writeAccount(LS.filterCollapsed, prefs.filterCollapsed);
@@ -346,8 +333,6 @@ export default function App() {
         onOpenAbout={() => setAboutOpen(true)}
         onChangeLanguage={changeLanguage}
         language={i18n.language as LangCode}
-        collapsed={navCollapsed}
-        onToggleCollapse={() => setNavCollapsed(!navCollapsed)}
       />
       {/* The active page's side rail (feed filters / plex filters / playlists list), if it has one.
           A floating collapsible column beside the nav rail; hidden while a channel page overlays the