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/AccountPanel.tsx

@@ -15,6 +15,7 @@ import { invalidateUserAccess } from "../lib/adminUsers";
 import type { AdminUserAccess, AdminUserRow } from "../lib/api/admin";
 import { Button } from "./ui/Button";
 import { SURFACE } from "./ui/surfaces";
+import { SectionLabel } from "./ui/Headings";
 
 // The admin's answer to "why can this person not get in?", and the place every recovery action for
 // one account lives (E12 · S5, design option C).
@@ -115,9 +116,9 @@ export default function AccountPanel({
         <LoadingState />
       ) : (
         <>
-          <div className="text-2xs uppercase tracking-wider text-muted mb-2">
+          <SectionLabel level={3} className="mb-2">
             {t("users.account.access")}
-          </div>
+          </SectionLabel>
           <div className={`${SURFACE.card} p-3 space-y-2 mb-5`}>
             <AccessRow
               icon={Mail}
@@ -192,9 +193,9 @@ export default function AccountPanel({
             )}
           </div>
 
-          <div className="text-2xs uppercase tracking-wider text-muted mb-2">
+          <SectionLabel level={3} className="mb-2">
             {t("users.account.recovery")}
-          </div>
+          </SectionLabel>
           <div className="space-y-2">
             {a.can_activate && (
               <Button