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/DownloadCenter.tsx

@@ -167,18 +167,20 @@ export default function DownloadCenter() {
 
   return (
     <>
-      {/* Page title + subtitle + tabs are fixed chrome; the tab content scrolls under them. */}
+      {/* The tabs are the page's band (B4); the tab content scrolls under them. No visible title: the
+          nav rail and the tab title name the page, the shell's hidden h1 names it to a screen reader. */}
       <PageToolbar>
-        <div className="px-4 pt-2 max-w-4xl w-full mx-auto">
-          <div className="flex items-center justify-between gap-3 mb-1">
-            <h1 className="text-xl font-semibold">{t("downloads.page.title")}</h1>
-            <Button variant="ghost" size="sm" icon={<Settings2 />} onClick={() => setManage(true)}>
-              {t("downloads.page.manage")}
-            </Button>
-          </div>
-          <p className="text-sm text-muted mb-4">{t("downloads.page.subtitle")}</p>
-
-          <Tabs tabs={tabs} active={tab} onChange={setTab} />
+        <div className="px-4 pt-2 pb-2 max-w-4xl w-full mx-auto flex flex-wrap items-center justify-between gap-2">
+          <Tabs tabs={tabs} active={tab} onChange={setTab} className="flex flex-wrap gap-1" />
+          <Button
+            variant="ghost"
+            size="sm"
+            icon={<Settings2 />}
+            onClick={() => setManage(true)}
+            className="shrink-0"
+          >
+            {t("downloads.page.manage")}
+          </Button>
         </div>
       </PageToolbar>