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.

docs/API.md

@@ -85,7 +85,7 @@ A dashboard shared by several people runs in **server mode** ([SERVER-MODE.md](S
 | `POST /api/auth/login` `{username, password}` | `{user}` + the cookie; `401 EBADLOGIN`, `429 ETOOMANY`, `404 ENOTSERVER` in local mode |
 | `POST /api/auth/logout` | `{ok: true}`, cookie cleared |
 
-In server mode a **developer** sees only rows of the workspaces they own (`lanes.owner_id`): every list, count, chart and cost answer is limited to them, and a request naming another user's session, agent, workspace or run (in the path, query or body) answers `404 ENOTFOUND`. Admin and analyst see everything; `GET /api/users` lists every account (`displayName`, `email`, `createdAt`, `logins`, `lastLoginAt` included) with its workspaces, sessions and 7/30-day `{cost, tokens}`, `GET /api/users/storage[?refresh=1]` each user's home size (`{storage: {<id>: {bytes, files, partial, measuredAt} | null}, measuring}` — measured in the background, see [SERVER-MODE.md](SERVER-MODE.md)), `GET /api/users/tmux` each member's own tmux server under one Linux user per account (`{available, installed, matched, users: [{id, username, running, stuck?, version, pid, startedAt, sessions, busy, outdated}]}`; an admin's `POST /api/users/:id/tmux-restart` and `POST /api/users/tmux-restart-idle` restart them with `currentPassword`, audited, `409 ETMUXSTUCK` for a server that will not stop), and `GET /api/users/audit[?limit=]` the staff reads of developers' sessions and admins' account changes (`reader`/`owner` are usernames, `reader_display`/`owner_display` their display names or null) (all `403 EADMIN` for a developer). See [SERVER-MODE.md](SERVER-MODE.md#what-each-user-sees).
+In server mode a **developer** sees only rows of the workspaces they own (`lanes.owner_id`): every list, count, chart and cost answer is limited to them, and a request naming another user's session, agent, workspace or run (in the path, query or body) answers `404 ENOTFOUND`. Admin and analyst see everything; `GET /api/users` lists every account (`displayName`, `email`, `createdAt`, `logins`, `lastLoginAt` included) with its workspaces, sessions and 7/30-day `{cost, tokens}`, `GET /api/users/storage[?refresh=1]` each user's home size (`{storage: {<id>: {bytes, files, partial, measuredAt} | null}, measuring}` — measured in the background, see [SERVER-MODE.md](SERVER-MODE.md)), `GET /api/users/tmux` each member's own tmux server under one Linux user per account (`{available, installed, matched, users: [{id, username, running, stuck?, version, pid, startedAt, sessions, busy, outdated}]}`; an admin's `POST /api/users/:id/tmux-restart` and `POST /api/users/tmux-restart-idle` restart them with `currentPassword`, audited, `409 ETMUXSTUCK` for a server that will not stop), `GET /api/host` the machine and each user's live share of it (`{at, hostname, os, uptime, cpu: {model, cores, load}, memory: {total, available, used, swapTotal, swapUsed}, disks: [{device, mount, type, kind: "HDD"|"SSD"|null, total, used, free}], homesMount, gpus: [{name, utilization, memoryUsed, memoryTotal}], usage: {cpu, sampleMs, groups: [{key, kind: "user"|"host"|"system", userId?, username?, cpu, memory, memoryPercent, processes, top: [{pid, name, cpu, memory}]}]} | null}` — read-only, cached 3 s, `usage` null off Linux), and `GET /api/users/audit[?limit=]` the staff reads of developers' sessions and admins' account changes (`reader`/`owner` are usernames, `reader_display`/`owner_display` their display names or null) (all `403 EADMIN` for a developer). See [SERVER-MODE.md](SERVER-MODE.md#what-each-user-sees).
 
 An admin changes accounts with `POST /api/users` (`{username, password, role}` → `201 {user}`), `GET /api/users/:id` (`{user, workspaces: [{id, title, cwd}]}`, the account with `logins` and `lastLoginAt`, for the Edit form; admin or analyst), `GET /api/users/:id/avatar` (that account's picture for the sidebar's owner groups — admin or analyst, `nosniff` + `sandbox` CSP, `404 ENOAVATAR`; a workspace in `GET /api/lanes` carries `owner_avatar_at` to version its URL), `PATCH /api/users/:id` (any of `{displayName?, email?, role?, disabled?, password?, addWorkspaces?: [laneId], removeWorkspaces?: [laneId], signOut?: true}` — every field validated before any is written; `removeWorkspaces` leaves the workspace with no owner and answers `409 ENOTOWNED` for one the user does not own; an admin's own `password`/`signOut` keeps the login it came from), `POST /api/users/:id/password` (`{password}`; that user's logins end) `POST /api/users/:id/workspaces` (`{laneId}`; the workspace's owner becomes that user) and `DELETE /api/users/:id` (delete for good: the user's logins, panes, workspaces' records, prefs and — under S3 — their Linux account and whole home, removed FIRST so a refusal (`502 EPROVISION`) leaves every row; `409 ESELF` for yourself, `409 ELASTADMIN` for the last admin; session history and the audit log stay). Every one carries `currentPassword`, the admin's own password: `403 EREAUTH` when it is wrong or missing, `429 ETOOMANY` after 10 wrong tries in 15 minutes. `409 ESELF` for an admin's own role or disable, `409 EEXISTS` for a taken name, `400 EBADUSER`/`EBADPASSWORD`/`EBADROLE`/`EBADEMAIL`/`EBADFIELD`, `404 ENOUSER`/`ENOLANE`; a developer or analyst gets `403 EADMIN`, and local mode `403 ELOCALMODE`. No answer carries a password hash; each change is written to the audit log.