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)

INSTALL.md

@@ -117,9 +117,10 @@ echo 'export PATH="$HOME/.local/bin:$PATH"' >> ~/.zshrc   # where the plugin put
   `node:sqlite`); a checkout still runs on Node 20. The `agenthub` launcher
   pins the node that installed it and falls back to `node` on `PATH` when a
   `brew upgrade` or `nvm` switch deletes that binary.
-- **Starts at login by itself (LaunchAgent).** The plugin bootstrap installs
-  `~/Library/LaunchAgents/io.smartgift.agenthub.plist` (a checkout: `agenthub
-  service install`), so the dashboard comes back after every login with no
+- **Starts at login once you turn it on (LaunchAgent).** Off by default; with
+  Settings → About → **Start AgentHub automatically** on, the plugin bootstrap
+  installs `~/Library/LaunchAgents/io.smartgift.agenthub.plist` (a checkout:
+  the same switch, or `agenthub service install`), so the dashboard comes back after every login with no
   Claude Code session, and launchd restarts it if it dies. Lane panes survive
   a restart of the server. Its output goes to `~/Library/Logs/agenthub.log`.
   A LaunchAgent needs a login at the Mac itself; over SSH only, or when