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/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