Admin UI: show gateway status and improve controls

[isaacrowntree]

Problem

The admin UI's Gateway Controls section only shows a "Restart Gateway" button with static text. There's no indication of whether the gateway is actually running, starting, or stopped.

This makes it hard to diagnose issues — for example, the sync endpoint returns 500 when the gateway isn't running (#207), but the admin UI gives no visual clue that the gateway is down. You have to check logs to understand what's happening.

After clicking "Restart Gateway", there's also no feedback on whether the restart succeeded. The button shows "Restarting..." briefly during the API call, but the restart is fire-and-forget (waitUntil) so the client never learns the outcome.

Proposal

Add a gateway status indicator and improve the controls:

1. New endpoint GET /api/admin/gateway/status returning process state (running / starting / stopped) using the existing findExistingMoltbotProcess() + a quick port check
2. Status badge next to "Gateway Controls" header — green "Running" / yellow "Starting" / red "Stopped"
3. Context-aware button — show "Start Gateway" when stopped, "Restart Gateway" when running
4. Post-restart polling — after restart, poll status until gateway is ready (or timeout with error)
5. Disable "Backup Now" when gateway is stopped, since sync requires the gateway (R2 backup never runs: requires active gateway process, creating chicken-and-egg problem #207)

Related

• R2 backup never runs: requires active gateway process, creating chicken-and-egg problem #207 — R2 backup fails when gateway isn't running (chicken-and-egg)
• Handle 'already in use' error when R2 bucket is already mounted #209 — adds ensureMoltbotGateway() before sync
• fix: add gateway check and diagnostics to sync endpoint #211 — adds gateway check + diagnostics to sync endpoint

[isaacrowntree]

Implementation submitted in #216.

Adds GET /api/admin/gateway/status endpoint and updates the admin UI with:

• Status badge (green Running / yellow pulsing Starting / red Stopped)
• Conditional button text (Start/Restart Gateway)
• Post-restart polling until running or 60s timeout
• Backup button disabled when gateway not running
• 6 new tests for the endpoint