feat(admin): Administration page — Users tab + Server tab (hardware, per-user CPU/memory/home share)

/users becomes /admin (redirected), a tab shell over the existing Users view
and a new Server tab backed by GET /api/host (server/lib/host-stats.js):
hostname/OS/uptime, CPU and load, memory and swap, every real disk with
HDD/SSD, NVIDIA GPUs, and each user's live share from two /proc samples —
attributed by the member's Linux uid (S3) or, for host processes, by the
inherited AGENTHUB_LANE_ID's workspace owner. Read-only, staff-only (/host
in STAFF_ONLY_PREFIXES), comm names only. Members' tmux card moves to the
Server tab.

client/src/App.tsx

@@ -89,7 +89,9 @@ const SnapshotView = lazy(() =>
   import("./pages/SnapshotView").then((m) => ({ default: m.SnapshotView }))
 );
 const FilesPage = lazy(() => import("./pages/FilesPage").then((m) => ({ default: m.FilesPage })));
-const Users = lazy(() => import("./pages/Users").then((m) => ({ default: m.Users })));
+const Administration = lazy(() =>
+  import("./pages/Administration").then((m) => ({ default: m.Administration }))
+);
 const Artifacts = lazy(() => import("./pages/Artifacts").then((m) => ({ default: m.Artifacts })));
 
 function PageFallback() {
@@ -237,13 +239,15 @@ export default function App() {
               }
             />
             <Route
-              path="users"
+              path="admin"
               element={
                 <Page>
-                  <Users />
+                  <Administration />
                 </Page>
               }
             />
+            {/* The Users page became Administration's first tab. */}
+            <Route path="users" element={<Navigate to="/admin" replace />} />
             <Route
               path="settings/*"
               element={