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)

README.md

@@ -203,7 +203,10 @@ agenthub service uninstall     # disable and remove it
 On Linux, `agenthub service install` also tries `loginctl enable-linger` so the service
 starts on a cold boot, before anyone logs in; if that needs elevated rights on
 your machine it prints the `sudo loginctl enable-linger <user>` command to run
-once yourself. Plugin installs get the same persistence automatically — see
+once yourself. The same is a switch in the dashboard — Settings → About →
+**Start AgentHub automatically** (off by default; it enables or disables the
+unit and never stops the running server). Plugin installs get it only once
+that switch is on — see
 [Auto-start on boot](docs/PLUGINS.md#auto-start-on-boot) in `docs/PLUGINS.md`.
 
 ## The CLI