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/ChannelDiscovery.tsx

@@ -200,7 +200,7 @@ export default function ChannelDiscovery({
     <>
       {/* Intro + the table controls are fixed chrome; only the table rows scroll (sticky header). */}
       <PageToolbar>
-        <div className="px-4 pt-3 max-w-384 mx-auto flex items-start justify-between gap-4">
+        <div className="px-4 pt-2 max-w-384 mx-auto flex items-start justify-between gap-4">
           <p className="text-xs text-muted leading-relaxed">{t("channels.discovery.intro")}</p>
           {/* Cards has no column headers, so a dropdown drives the sort + a pager (table uses its own). */}
           {layout === "cards" && (