feat(admin): fix the tab bar on Users + Configuration pages

Apply the fixed-chrome pattern to the two admin pages with a horizontal
tab bar: the tabs (and Config's intro) pin in the shell's fixed band while
the tab content scrolls under them. Config's floating save bar is
unaffected. Suite 15/15, tsc + knip clean.

frontend/src/components/ConfigPanel.tsx

@@ -6,6 +6,7 @@ import { api, type ConfigItem, type PlexTestResult } from "../lib/api";
 import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
 import Tabs, { usePersistedTab } from "./Tabs";
+import { PageToolbar } from "./PageShell";
 import { Switch } from "./ui/form";
 import { DraftSaveBar, type SaveState } from "./ui/DraftSaveBar";
 import { LS } from "../lib/storage";
@@ -151,11 +152,15 @@ export default function ConfigPanel() {
   }));
 
   return (
-    <div className="p-4 max-w-3xl w-full mx-auto pb-24">
-      <p className="text-xs text-muted mb-4 leading-relaxed">{t("config.intro")}</p>
-
-      <Tabs tabs={groupTabs} active={activeGroup} onChange={setTab} />
-
+    <>
+      {/* Intro + tab bar are fixed chrome; the config fields scroll under them. */}
+      <PageToolbar>
+        <div className="px-4 pt-3 max-w-3xl w-full mx-auto">
+          <p className="text-xs text-muted mb-3 leading-relaxed">{t("config.intro")}</p>
+          <Tabs tabs={groupTabs} active={activeGroup} onChange={setTab} />
+        </div>
+      </PageToolbar>
+      <div className="px-4 pb-24 pt-3 max-w-3xl w-full mx-auto">
       {activeGroup && (
         <div className="glass rounded-2xl p-4 mb-4">
           <div className="divide-y divide-border/60">
@@ -264,6 +269,7 @@ export default function ConfigPanel() {
         }}
       />
     </div>
+    </>
   );
 }