feat(server-s3): one Linux user per account (S3) — experimental

Server mode can now run each account as its own Linux user (ah-<name>)
when `agenthub server install-system` has set S3 up. The dashboard
reaches a member only through sudo -u / write-as / pane-launch, reads a
member's files only through owner- and type-checked opens, and hooks
are authenticated by peer uid. Local mode and server mode without S3
are unchanged.

EXPERIMENTAL: tested with fake sudo and fake accounts only. It has not
yet been verified on a real install with real member accounts, sudo or
a real Codex login. Do not offer it to real users yet.

- S3 core: provisioning, per-member tmux servers, the proxy key
  gateway, shared tools in /opt, dropped capabilities for children
  (P0–P5)
- P4: plan and quota per member home
- P6: ~/.claude.json seed; opt-in plugin baseline installed as the
  member; managed-settings spike script (not run)
- P7: Codex account pool (`agenthub accounts add --tool codex`)
- Roles merged with main's admin / analyst / developer: analysts write
  only their own rows, under S3 as well
- Per-account prefs: one implementation (Chat UI + onboarding)
- Security reviews F1–F49 (five rounds), each fix with a regression
  test: symlink/TOCTOU-safe member reads, no host writes in a member
  home, credentials and the data dir never served, owner and folder
  account must agree, pane tokens bound to the pane's uid, bounded
  sudo/tmux/git calls and member reads (no hang or crash from a member),
  async route rejections answer 500 instead of exiting
- Docs: docs/SERVER-MODE.md (S3 model, accepted ceilings, operator
  runbook), docs/plans/server-mode/S3-PLAN.md, CLAUDE.md S3 rules

Tests: server 2838/2841 (3 need 127.0.0.1:3000, held on this box);
client 1162/1162; tsc clean; headers check passes.

Co-Authored-By: Claude <noreply@anthropic.com>

client/src/components/AuthGate.tsx

@@ -8,6 +8,8 @@
  * (`AUTH_REQUIRED_EVENT`). Logging out unmounts the app, so no page keeps the
  * previous user's data. `useAuth()` gives the sidebar the user and `logout`;
  * `usePermissions()` says what that user's role may do, so pages hide the rest.
+ * Once an account is known (local mode is one), its own UI preferences are
+ * loaded into this browser (`lib/userPrefs.ts`).
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
@@ -21,6 +23,7 @@ import {
 } from "react";
 import { useTranslation } from "react-i18next";
 import { api, AUTH_REQUIRED_EVENT, type AuthState, type AuthUser } from "../lib/api";
+import { hydrate } from "../lib/userPrefs";
 
 type AuthValue = { state: AuthState; logout: () => Promise<void> };
 
@@ -114,6 +117,18 @@ export function AuthGate({ children }: { children: ReactNode }) {
     };
   }, []);
 
+  // The signed-in account's own preferences (lib/userPrefs.ts): loaded once
+  // per account, so a second login in this browser brings that account's.
+  const account =
+    state?.mode === "local"
+      ? "local"
+      : state?.mode === "server" && state.user
+        ? `u${state.user.id}`
+        : null;
+  useEffect(() => {
+    if (account) void hydrate();
+  }, [account]);
+
   if (!state) return null;
   if (state.mode === "server" && !state.user) {
     return <LoginPage onLoggedIn={(user) => setState({ mode: "server", user })} />;