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/Feed.tsx

@@ -568,7 +568,7 @@ export default function Feed({
   }));
 
   const toolbar = (
-    <div className="pb-3">
+    <div className="pb-1.5">
       <div className="flex items-center gap-1.5 gap-y-2 flex-wrap">
         {/* Watch state is single-select (always exactly one); content types are an independent
             multi-select. Both compress to one labeled dropdown pill via FilterSelect. */}
@@ -673,7 +673,7 @@ export default function Feed({
   );
 
   return (
-    <div className={channelScoped ? "px-4 sm:px-6 pb-4" : "px-4 pb-4 pt-2"}>
+    <div className={channelScoped ? "px-4 sm:px-6 pb-4" : "px-4 pb-4 pt-1"}>
       {/* The toolbar (+ active-filter chips) is FIXED chrome: it portals into a PageToolbarSlot so
           it stays put while the cards scroll (bug 4). On the feed page that slot is the shell's top
           band; on a channel page ChannelPage overrides the slot with its own sticky sub-header, so
@@ -681,7 +681,7 @@ export default function Feed({
           baseline (channel-scoped, whole-catalog, show-all), so each would read as a "filter you
           applied" when it's just the page being itself. */}
       <PageToolbar>
-        <div className={channelScoped ? "px-4 sm:px-6" : "px-4 pt-4 pb-1"}>
+        <div className={channelScoped ? "px-4 sm:px-6" : "px-4 pt-2 pb-1"}>
           {toolbar}
           {!channelScoped && (
             <ActiveFilterChips