Heartbeat: keep a box visible when its tunnel is dead (alive-without-URL heartbeat)

[KrasimirKralev]

Follow-up from the #326 code review (finding #2).

Context

#326 makes the heartbeat tick refuse to advertise a dead tunnel hostname and restart clawbox-tunnel instead. That's the right call for the common case (restart mints a fresh URL the next tick pushes).

The gap

On a dead verdict the tick does not call pushHeartbeatTick. If the restart keeps failing (Cloudflare can't mint a tunnel, or the service won't restart), the box sends no heartbeat and the portal flips it Offline after the staleness window — the alive box disappears from fleet ops entirely, instead of showing Online-with-no-working-link.

pushHeartbeatTick(url) no-ops on a null URL, so there's no way today to say "box is alive, tunnel is not."

Proposed fix

Add an alive-without-URL heartbeat: push a liveness signal that carries box state (lastSeenAt) but explicitly no/blank tunnel URL, and have the portal render that as Online — Remote Access unavailable rather than a dead link or Offline. Then the dead branch can push that while it restarts.

This is a portal-protocol change (box + portal side), which is why it was scoped out of #326.

[github-actions]

🦀 ClawReview

Scuttled over to help sort this one 🦀

Add heartbeat signal when box is alive but tunnel is dead to keep it visible in fleet ops.

At a glance

• Category: enhancement · Area: gateway
• Priority: 🟡 medium

Suggested next step: Implement alive-without-URL heartbeat protocol in both box and portal; push liveness signal on tunnel restart failure so box shows as Online—Remote Access unavailable.

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