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/Stats.tsx

@@ -13,6 +13,7 @@ import Tooltip from "./ui/Tooltip";
 import { LoadingState, StateMessage } from "./ui/QueryState";
 import FilterChipGroup from "./ui/FilterChipGroup";
 import Tabs from "./ui/Tabs";
+import { PageToolbar } from "./ui/PageShell";
 import { Section, SettingRow } from "./ui/form";
 import { useMe } from "../lib/useMe";
 import { notifyActionError } from "../lib/actionErrors";
@@ -35,17 +36,23 @@ export default function Stats() {
   const tab: StatsTab = tabRaw === "system" && isAdmin ? "system" : "overview";
 
   return (
-    <div className="p-4 max-w-4xl mx-auto">
-      {/* Only admins have a second (System) tab; everyone else just sees the Overview. */}
+    <div className="px-4 pb-4 pt-3 max-w-4xl mx-auto">
+      {/* Only admins have a second (System) tab; everyone else just sees the Overview. The tabs are
+          the page's band (B4), so they stay in reach on the long System tab. */}
       {isAdmin && (
-        <Tabs
-          active={tab}
-          onChange={setTab}
-          tabs={(["overview", "system"] as StatsTab[]).map((id) => ({
-            id,
-            label: t(`stats.tabs.${id}`),
-          }))}
-        />
+        <PageToolbar>
+          <div className="px-4 pt-2 max-w-4xl w-full mx-auto">
+            <Tabs
+              active={tab}
+              onChange={setTab}
+              tabs={(["overview", "system"] as StatsTab[]).map((id) => ({
+                id,
+                label: t(`stats.tabs.${id}`),
+              }))}
+              className="flex flex-wrap gap-1 pb-2"
+            />
+          </div>
+        </PageToolbar>
       )}
 
       {tab === "system" && isAdmin ? <SystemStats /> : <Overview me={me} />}