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/__tests__/screens.snapshot.test.tsx

@@ -334,6 +334,10 @@ vi.mock("../../lib/api", async (importOriginal) => {
           rotate: r({ token: "t", enabled: true, env_owned: false }),
           revoke: r({ enabled: false, env_owned: false }),
         },
+        autostart: {
+          get: r({ available: true, kind: "systemd", enabled: false, foreign: false }),
+          set: r({ available: true, kind: "systemd", enabled: true, foreign: false }),
+        },
         privacy: {
           get: r({
             redact_secrets: false,