style(layout): tighten the oversized vertical gaps around the page toolbars

The floating-header → toolbar → filter-chips → content stack carried ~2× the whitespace it
needed. Halve it, app-wide where the chrome is shared:
- PageShell: the --hdr-h gap below the floating header 8px → 4px (every page's header).
- Feed: toolbar wrapper pt-4 → pt-2, the toolbar's own pb-3 → pb-1.5, and the grid's top pt-2
  → pt-1. Measured on the feed: header→toolbar 24→12px, toolbar→chips 12→6px, chips→cards 12→8px.
- Plex / Playlists / ChannelDiscovery / DownloadCenter / Notifications / AdminUsers: toolbar
  wrapper pt-3 → pt-2, so every page's top band matches the tighter feed.

PageShell.dom.test updated for the 4px gap.

frontend/src/components/DownloadCenter.tsx

@@ -135,7 +135,7 @@ export default function DownloadCenter() {
     <>
       {/* Page title + subtitle + tabs are fixed chrome; the tab content scrolls under them. */}
       <PageToolbar>
-        <div className="px-4 pt-3 max-w-4xl w-full mx-auto">
+        <div className="px-4 pt-2 max-w-4xl w-full mx-auto">
           <div className="flex items-center justify-between gap-3 mb-1">
             <h1 className="text-xl font-semibold">{t("downloads.page.title")}</h1>
             <button