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/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. */}