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/ExcludedChannelList.tsx

@@ -4,6 +4,7 @@ import NavGrid from "./ui/NavGrid";
 import { navItemAttrs } from "../lib/listNav";
 import { Button } from "./ui/Button";
 import { SURFACE } from "./ui/surfaces";
+import { SectionLabel } from "./ui/Headings";
 
 // The presentation shared by the channel manager's "excluded channel" tabs — Hidden and Blocked.
 // A responsive grid of proper channel cards (avatar + name + open-on-YouTube, via ChannelLink) each
@@ -42,7 +43,7 @@ export default function ExcludedChannelList({
     <>
       <div className="flex items-center gap-2 mb-3">
         {icon}
-        <span className="text-xs uppercase tracking-wide text-muted">{title}</span>
+        <SectionLabel>{title}</SectionLabel>
       </div>
       <NavGrid className="grid grid-cols-1 gap-2 sm:grid-cols-2 lg:grid-cols-3">
         {channels.map((c, i) => (