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/i18n/locales/en/settings.json

@@ -303,6 +303,14 @@
     "typeToConfirm": "Type “{{phrase}}” to confirm:",
     "phrase": "clear all data"
   },
+  "autostart": {
+    "title": "Startup",
+    "label": "Start AgentHub automatically",
+    "desc_systemd": "Run the dashboard as a systemd --user service, so it starts at boot without Claude Code being opened. Turning it off keeps the running server up.",
+    "desc_launchd": "Run the dashboard as a LaunchAgent, so it starts at login without Claude Code being opened. Turning it off keeps the running server up.",
+    "unavailable": "Not available here: needs systemd --user on Linux, or a GUI login on macOS.",
+    "foreign": "A service named agenthub that AgentHub did not write already exists — AgentHub leaves it alone."
+  },
   "about": {
     "title": "About",
     "description": "Server runtime information.",