feat(layout): centre the header on each page's content width, centre the playlist table

Every page's floating header (search bar + sync chip) and the toolbar band now centre on the SAME
max-width the content column uses, instead of spanning the whole column while a max-w page centres
its grid. So the search bar lines up with the left edge of the cards/table on every page, and sits
centred with the content — not against the full page width.

- PageShell takes a `maxWidth`, published as --content-mw; the header adds max-w-[--content-mw] mx-auto
  inside its left-4 / right-(1rem+scrollbar) span, so it centres on that width (`none` = full width).
- moduleRegistry PAGE_CONTENT carries each page's content max width (plex 1600px, channels 96rem,
  playlists 64rem, downloads 56rem, notifications/users 48rem; feed stays full width) — one source.
- Playlists: its content (header, filter row, table) was left-aligned max-w-5xl → now mx-auto, so the
  table is centred and the header lines up with it.

Verified at 1920px: header left edge == content left edge — feed 340, channels 213, playlists 487.

frontend/src/App.tsx

@@ -313,7 +313,7 @@ export default function App() {
   const me = meQuery.data!;
   const pageGate = moduleDef(page).gate;
   const activePage: Page = !pageGate || pageGate(me) ? page : "feed";
-  const { Component: PageContent, fadeTop } = PAGE_CONTENT[activePage];
+  const { Component: PageContent, fadeTop, maxWidth } = PAGE_CONTENT[activePage];
   const railDef = channelView ? undefined : PAGE_RAIL[activePage];
 
   return (
@@ -379,6 +379,7 @@ export default function App() {
         ) : (
           <PageShell
             scrollKey={normalScrollKey}
+            maxWidth={maxWidth}
             // Channels: the registry disables the top fade for its sticky-header table, but the card
             // layout has no sticky header, so re-enable it there.
             fadeTop={activePage === "channels" ? channelLayout === "cards" : (fadeTop ?? true)}

frontend/src/components/Header.tsx

@@ -89,9 +89,11 @@ const Header = forwardRef<
     // slim and/or no filter sidebar is present — no hard-coded offset.
     <div
       ref={ref}
-      // left-4 / right = 1rem + the scrollbar gutter: the header spans exactly the card grid's width
+      // left-4 / right = 1rem + the scrollbar gutter defines the full content column; max-w-[--content-mw]
-      // (left card's left edge to right card's right edge), matching the toolbar band below it.
+      // + mx-auto then centres the header on the SAME width the page's content uses, so on a max-width page
-      className="pointer-events-none absolute top-3 left-4 right-[calc(1rem+var(--sbw))] z-chrome flex flex-wrap items-center gap-x-3 gap-y-2"
+      // (Plex/Channels/Playlists) the header lines up with the centred grid, not the whole column. `none`
+      // (feed) = full width. Matches the toolbar band below it.
+      className="pointer-events-none absolute top-3 left-4 right-[calc(1rem+var(--sbw))] mx-auto max-w-[var(--content-mw,none)] z-chrome flex flex-wrap items-center gap-x-3 gap-y-2"
     >
       {search}
       {/* Library scope (movie/show/both) sits right of the search box on Plex — relocated here from

frontend/src/components/PageShell.tsx

@@ -52,6 +52,7 @@ export default function PageShell({
   children,
   scrollKey,
   fadeTop = true,
+  maxWidth,
 }: {
   header?: ReactNode;
   banners?: ReactNode;
@@ -60,8 +61,20 @@ export default function PageShell({
   scrollKey?: string;
   // Off when this page's scroll region starts with a sticky header — see PageScroller.
   fadeTop?: boolean;
+  // The content column's max width (e.g. "1600px"); the header + toolbar centre on the same width via
+  // --content-mw. Omit for a full-width page (feed). See index.css --content-mw.
+  maxWidth?: string;
 }) {
   const [toolbarSlot, setToolbarSlot] = useState<HTMLDivElement | null>(null);
+  // Publish this page's content max-width so the floating header (and toolbar band) can centre on the
+  // same width the content column uses. Global (one page shell renders at a time).
+  useLayoutEffect(() => {
+    const root = document.documentElement;
+    root.style.setProperty("--content-mw", maxWidth ?? "none");
+    return () => {
+      root.style.removeProperty("--content-mw");
+    };
+  }, [maxWidth]);
   // The floating header pads the content below it by `--hdr-h`. That must equal the header's REAL
   // height, which grows as the flex-wrap header wraps to 2–3 rows on a narrow window — a fixed pad
   // would let the content toolbar slide up under it (E5 S7). Measure the header and write its height

frontend/src/components/Playlists.tsx

@@ -571,7 +571,7 @@ export default function Playlists() {
       {/* Header (title, actions, view controls) is fixed chrome — it portals into the shell's
           fixed band, so only the video list scrolls. */}
       <PageToolbar>
-        <div className="px-4 pt-2 pb-2">
+        <div className="px-4 pt-2 pb-2 max-w-5xl mx-auto">
           <div className="flex gap-3 items-start mb-4">
             <div className="w-24 h-13.5 rounded-lg bg-card overflow-hidden shrink-0">
               {items[0]?.thumbnail_url && (
@@ -757,7 +757,7 @@ export default function Playlists() {
           accessibility={{ screenReaderInstructions, announcements }}
         >
           {/* Not clipped, so the header's filter popovers can overflow the table box. */}
-          <div className="max-w-5xl">
+          <div className="max-w-5xl mx-auto">
             {/* table-fixed, not auto: every column but the title has a declared width, so the
                 title absorbs the rest and its `truncate` works. Auto layout sizes columns from
                 min-content, and a nowrap title cell's min-content is the entire title. */}

frontend/src/components/moduleRegistry.tsx

@@ -75,21 +75,25 @@ export type PageDef = {
   // Channels + Audit render a table with a sticky header at the scroller top; the shell's top fade
   // would sit on it and flip on with the first scroll, so they turn it off. Defaults to on.
   fadeTop?: boolean;
+  // The content column's max width. The header + toolbar centre on this same width (via --content-mw)
+  // so the floating chrome lines up with the centred content instead of spanning the whole column.
+  // MUST match the max-w the page's own content wrapper uses. Omit for a full-width page (feed).
+  maxWidth?: string;
 };
 
 // page id → the component App renders in the content column (+ its shell chrome config).
 export const PAGE_CONTENT: Record<Page, PageDef> = {
   feed: { Component: FeedPage },
-  channels: { Component: Channels, fadeTop: false },
+  channels: { Component: Channels, fadeTop: false, maxWidth: "96rem" },
-  playlists: { Component: Playlists },
+  playlists: { Component: Playlists, maxWidth: "64rem" },
-  plex: { Component: PlexBrowse },
+  plex: { Component: PlexBrowse, maxWidth: "1600px" },
-  notifications: { Component: NotificationsPanel },
+  notifications: { Component: NotificationsPanel, maxWidth: "48rem" },
   messages: { Component: MessagesPage },
-  downloads: { Component: DownloadCenter },
+  downloads: { Component: DownloadCenter, maxWidth: "56rem" },
   stats: { Component: Stats },
   scheduler: { Component: Scheduler },
   config: { Component: ConfigPanel },
-  users: { Component: AdminUsers },
+  users: { Component: AdminUsers, maxWidth: "48rem" },
   audit: { Component: AuditLog, fadeTop: false },
   settings: { Component: SettingsPanel },
 };

frontend/src/index.css

@@ -104,6 +104,11 @@
      Defaults to 0 for overlay-scrollbar platforms where the gutter is empty. */
   --sbw: 0px;
 
+  /* Max width of a page's content column (cards / table). The floating header + toolbar band read it
+     so they centre on the SAME max-width the content uses, instead of spanning the whole column while
+     a max-w page centres its grid. Each page sets it via PageShell's `maxWidth`; `none` = full width. */
+  --content-mw: none;
+
   /* ===== Glass tunables — every value that materially drives the look lives here so it can be
      tuned in ONE place. ===== */
   /* GLOBAL baseline = "Adaptive": solid-enough dark glass that reads well on content-less chrome