Be aware of being online or offline

[t-mart]

Because Mousehole often runs as a sidecar onto other container's networks, people frequently run into problems when that other container stops and Mousehole is still attached to its network, but it has no VPN/routes/etc.

We should write ephemeral state into Mousehole's backend that keeps track of "being online", which I will define as if the last fetch to MAM threw an exception or not. Example:

// connectivitiy.ts
export let isOnline = true;

// some-handler.ts
try {
  const response = await fetch(endpointUrl, { ... });
  isOnline = true;
  // ... rest of existing logic (parse body, validate schema, return)
} catch (error) {
  isOnline = false;
  throw error;
}

Again, this should be ephemeral state, so we do not need to store it.

After we get this plumbed in, we should surface it in:

• server logs immediately when it happens
• the GET /state API endpoint
• the front end, maybe in a new row? in the status section
• websocket messages?
• also, the GET /ok endpoint should incorporate this.

Also, update the error documentation with this.

[t-mart]

Completed in 7d3a16d