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/plexViews.tsx

@@ -30,6 +30,7 @@ import NavGrid from "./ui/NavGrid";
 import { Chip } from "./ui/Chip";
 import { Pressable } from "./ui/Pressable";
 import { SURFACE } from "./ui/surfaces";
+import { PageHeading } from "./ui/Headings";
 
 // The Plex sub-router's view branches, split out of PlexBrowse. Each is invoked by PlexBrowse with
 // shell callbacks as props (onBack/onPlay/onFilter/…) — no view reaches back into the shell's state,
@@ -199,7 +200,7 @@ export function PlexPersonView({
                 )}
               </div>
               <div className="min-w-0 flex-1">
-                <h1 className="text-xl font-semibold">{d.name}</h1>
+                <PageHeading className="text-xl font-semibold">{d.name}</PageHeading>
                 {d.known_for_department && (
                   <p className="text-xs font-medium text-accent">{d.known_for_department}</p>
                 )}
@@ -393,7 +394,7 @@ export function PlexShowView({
               />
             }
           >
-            <h1 className="text-2xl font-semibold leading-tight">{show.title}</h1>
+            <PageHeading className="text-2xl font-semibold leading-tight">{show.title}</PageHeading>
             <div className="mt-1 flex flex-wrap items-center gap-x-2 gap-y-0.5 text-sm text-muted">
               {show.year != null && (
                 <Filterable
@@ -655,7 +656,7 @@ export function PlexSeasonView({
             <Button variant="link-muted" onClick={onBack} className="text-sm">
               {d.show.title}
             </Button>
-            <h1 className="text-2xl font-semibold leading-tight">{se.title}</h1>
+            <PageHeading className="text-2xl font-semibold leading-tight">{se.title}</PageHeading>
             <p className="text-sm text-muted mt-1">
               {t("plex.series.episodeCount", { count: se.episode_count })}
             </p>