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/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 card's left edge to right card's right edge), matching the toolbar band below it.
-      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"
+      // left-4 / right = 1rem + the scrollbar gutter defines the full content column; max-w-[--content-mw]
+      // + mx-auto then centres the header on the SAME width the page's content uses, so on a max-width page
+      // (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