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/i18n/locales/en/admin.json

@@ -0,0 +1,37 @@
+{
+  "title": "Administration",
+  "description": "Accounts and the server AgentHub runs on.",
+  "tabs": {
+    "users": "Users",
+    "server": "Server"
+  },
+  "server": {
+    "title": "Server",
+    "uptime": "up {{v}}",
+    "uptimeDays": "{{d}} d {{h}} h",
+    "uptimeHours": "{{h}} h {{m}} min",
+    "cpu": "CPU",
+    "cores": "{{n}} cores · load {{load}}",
+    "cpuHelp": "Share of the whole machine: 100% means every core is busy.",
+    "memory": "Memory",
+    "swap": "Swap",
+    "homesDisk": "Homes disk",
+    "homesDiskHelp": "The disk that holds the users' home folders.",
+    "free": "free",
+    "usageTitle": "Usage by user",
+    "usageHelp": "Live, sampled over {{s}} s. A process counts for a user when it runs as their Linux account or inside one of their workspaces' sessions. Memory is resident (RSS); shared pages count once per process. Click a row for its busiest processes.",
+    "usageUnavailable": "Per-user usage is read from /proc and is only available on Linux.",
+    "who": "User",
+    "processes": "Processes",
+    "home": "Home",
+    "sampling": "Sampling…",
+    "hostGroup": "Host account (AgentHub and other host processes)",
+    "systemGroup": "System (root and other accounts)",
+    "disks": "Disks",
+    "mount": "Mount",
+    "device": "Device",
+    "used": "Used",
+    "homesBadge": "homes",
+    "gpus": "GPUs"
+  }
+}