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/App.tsx

@@ -32,6 +32,8 @@ import Header from "./components/Header";
 import NavSidebar from "./components/NavSidebar";
 import BackToTop from "./components/ui/BackToTop";
 import PageShell from "./components/ui/PageShell";
+import { SkipLink } from "./components/ui/SkipLink";
+import { pageTitleKey } from "./lib/pageMeta";
 import ChatDock from "./components/ChatDock";
 import DebugRecorder from "./components/DebugRecorder";
 import Toaster from "./components/ui/Toaster";
@@ -367,7 +369,8 @@ export default function App() {
   const railDef = channelView ? undefined : PAGE_RAIL[activePage];
 
   return (
-    <div className="h-screen flex text-fg">
+    <div className="h-dvh flex text-fg">
+      <SkipLink label={t("nav.skipToContent")} />
       <NavSidebar
         me={me!}
         onOpenAbout={() => setAboutOpen(true)}
@@ -428,6 +431,7 @@ export default function App() {
           </PageShell>
         ) : (
           <PageShell
+            title={t(pageTitleKey(activePage))}
             scrollKey={normalScrollKey}
             maxWidth={maxWidth}
             header={<Header me={me!} onYtSearch={enterYtSearch} />}