feat: start-at-login is a Settings switch, off by default (v2.5.0)

- Settings → About → "Start AgentHub automatically" (AutostartSettings.tsx,
  GET/PUT /api/settings/autostart, server/lib/autostart.js). The switch is
  the systemd --user unit's / LaunchAgent's enabled state: on writes a unit
  for the running server when none exists and enables it, off disables it;
  neither starts or stops the server. A foreign unit is never touched
  (409 EFOREIGN)
- The plugin bootstrap no longer installs a unit on every install: it
  installs or starts one only while the user has it enabled, and a rewrite
  keeps the enabled state (writeUserService({ enable })). An install that
  already had the unit enabled stays on. A stale-but-enabled unit beside a
  live detached server is repointed, no longer started onto the bound port
- unitIsEnabled / setUnitEnabled in systemd-service.js and launchd-service.js

Also:
- fix(workspace): status bar no longer covers the chat box below md (#61)

client/src/pages/Settings.tsx

@@ -1,6 +1,6 @@
 /**
  * @file Settings.tsx
- * @description The install-wide settings, as three pages of the Settings nav (`SettingsHub.tsx`): `section="data"` — total cost, database overview, workspace housekeeping (dry run first, confirm before a real run), import, remote sources, push ingest (`components/settings/PushIngestSettings.tsx`), privacy (`components/settings/PrivacySettings.tsx`), export, this browser's preferences backup, session cleanup (whole days ≥ 1, a dry-run count in its confirm) and the danger zone (typed confirmation) —, `section="about"` — system information — and `section="notifications"` — browser notifications (per-browser, with a reset) and alerts (server-wide). Server info (`/api/settings/info`) is read only by Data and About, refreshed on activity at most every 10 s. Language, theme and the pet live on the Appearance page (`components/settings/AppearanceSettings.tsx`); per-tool settings live in that tool's page under `/settings/agents/<id>`.
+ * @description The install-wide settings, as three pages of the Settings nav (`SettingsHub.tsx`): `section="data"` — total cost, database overview, workspace housekeeping (dry run first, confirm before a real run), import, remote sources, push ingest (`components/settings/PushIngestSettings.tsx`), privacy (`components/settings/PrivacySettings.tsx`), export, this browser's preferences backup, session cleanup (whole days ≥ 1, a dry-run count in its confirm) and the danger zone (typed confirmation) —, `section="about"` — the start-at-login switch (`AutostartSettings`) and system information — and `section="notifications"` — browser notifications (per-browser, with a reset) and alerts (server-wide). Server info (`/api/settings/info`) is read only by Data and About, refreshed on activity at most every 10 s. Language, theme and the pet live on the Appearance page (`components/settings/AppearanceSettings.tsx`); per-tool settings live in that tool's page under `/settings/agents/<id>`.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 /* =============================================================================
@@ -107,6 +107,7 @@ import { ImportHistory } from "../components/ImportHistory";
 import { RemoteSources } from "../components/RemoteSources";
 import { PushIngestSettings } from "../components/settings/PushIngestSettings";
 import { PrivacySettings } from "../components/settings/PrivacySettings";
+import { AutostartSettings } from "../components/settings/AutostartSettings";
 import { Skeleton } from "../components/Skeleton";
 import { AlertsNotifications } from "../components/AlertsNotifications";
 import {
@@ -1121,6 +1122,9 @@ export function Settings({
 
       {section === "about" && (
         <section id="about" className="scroll-mt-24">
+          <div className="mb-4">
+            <AutostartSettings />
+          </div>
           <p className="text-xs text-fg-muted mb-4">{t("about.description")}</p>
           {sysInfo ? (
             <div className="card p-5">