feat(design): the page band (B4) — one h1 per page, section headings, skip to content, filters fold on a short window (E28 S3d4)

- PageShell names the page with a visually hidden h1; a view with its own title renders it as
  PageHeading and the hidden one steps aside (contract: no hand <h1> inside the shell).
- Section captions are SectionLabel headings (h2, h3 in a dialog), ui/form Section included.
- "Skip to content" is the first Tab stop and focuses the scroller's <main>.
- Below 600px of height the band's filter row folds into "Filters · n" (Feed, Plex, server log),
  the same row shown in place.
- The band holds controls only: Downloads and Notifications lose their title cards, the Config and
  channel-discovery intros scroll with the content, the Stats tabs and the audit pager join the band,
  and the server log's toolbar sticks with the fade under it.

frontend/src/components/ConfigPanel.tsx

@@ -187,14 +187,15 @@ export default function ConfigPanel() {
 
   return (
     <>
-      {/* Intro + tab bar are fixed chrome; the config fields scroll under them. */}
+      {/* The tab bar is the page's band (B4); the intro scrolls with the fields — in the band it
+          took three lines of a short window for text read once. */}
       <PageToolbar>
         <div className="px-4 pt-3 max-w-3xl w-full mx-auto">
-          <p className="text-xs text-muted mb-3 leading-relaxed">{t("config.intro")}</p>
           <Tabs tabs={groupTabs} active={activeGroup ?? ""} onChange={setTab} />
         </div>
       </PageToolbar>
-      <div className="px-4 pb-24 pt-3 max-w-3xl w-full mx-auto">
+      <div className="px-4 pb-24 pt-1 max-w-3xl w-full mx-auto">
+        <p className="text-xs text-muted mb-3 leading-relaxed">{t("config.intro")}</p>
         {activeGroup && (
           <div className={`${SURFACE.panel} p-4 mb-4`}>
             <div className="divide-y divide-border/60">