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/Workspace.tsx

@@ -925,10 +925,11 @@ export function Workspace() {
     <div
       className={
         sessionView
-          ? // Cancels Layout's page padding; the 1.5rem left is the status bar's.
-            "-m-5 flex h-[calc(100vh-1.5rem)] flex-col lg:-m-6"
+          ? // Cancels Layout's page padding; the 1.5rem left is the status bar's,
+            // plus the drawer's 2.75rem menu bar below `md` (issue #61).
+            "-m-5 flex h-[calc(100vh-1.5rem)] flex-col max-md:h-[calc(100vh-4.25rem)] lg:-m-6"
           : viewportLocked
-            ? "h-[calc(100vh-4rem)] lg:h-[calc(100vh-4.5rem)] flex flex-col gap-5"
+            ? "h-[calc(100vh-4rem)] max-md:h-[calc(100vh-6.75rem)] lg:h-[calc(100vh-4.5rem)] flex flex-col gap-5"
             : "space-y-5"
       }
     >