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/ChannelPage.tsx

@@ -39,6 +39,7 @@ import { useChannelTagActions } from "../lib/useChannelTagActions";
 import { Button } from "./ui/Button";
 import { IconButton } from "./ui/IconButton";
 import { IconLink } from "./ui/LinkButton";
+import { PageHeading } from "./ui/Headings";
 
 // A dedicated channel page: an "About"-style header (banner, avatar, stats, subscribe) over the
 // channel's videos (the catalog filtered to this channel). For an un-subscribed channel it
@@ -379,9 +380,9 @@ export default function ChannelPage({
             />
             <div className="flex-1 min-w-0 pb-1">
               <div className="flex items-center gap-2 flex-wrap">
-                <h1 data-testid="channel-title" className="text-xl font-semibold truncate">
+                <PageHeading data-testid="channel-title" className="text-xl font-semibold truncate">
                   {name}
-                </h1>
+                </PageHeading>
                 {ch?.blocked ? (
                   <span className="text-xs px-2 py-0.5 rounded-full bg-danger/15 text-danger">
                     {t("channel.blockedBadge")}