Say when a session's machine is offline, and keep its last screen

[Alexgodoroja]

The bug

A viewer can be perfectly connected to a session whose machine is not there, and be shown nothing at all.

Reported as "the terminal fails to appear, both in the app and in the standalone link". Reproduced on https://shell.online/s/2cMN…: the relay says {"exists":true,"status":"disconnected"}, two viewers are attached, the header dims a dot to "Offline", and the terminal area is a blank white rectangle with no explanation. The machine hosting it had rebooted; the session lives on the relay for the rest of its twelve hours, so every viewer since gets that page.

The relay has always reported the machine's state on every status message. Neither client acted on it:

• Standalone page — showed the grey "Offline" dot and nothing else. It has notices for a missing session, an ended one and a full one; not for a machine that is away.
• App pane — handleControl read only status === "exited" and dropped the rest, so the pane stayed on connected and drew an empty terminal with no banner at all.

That is what a reboot, a closed lid, a flat battery or a lost network looks like from the browser today.

The fix

One module, shared/host-presence.ts, vendored into the app the way protocol.ts and terminal-grid.ts already are, so both surfaces say the same thing in the same words.

Relay (worker/index.ts)

• Reports hostLastSeenAt and lastScreenAt alongside the status.
• Keeps the last full screen a host sent, chunked under one storage prefix because a snapshot (up to 512 KB) is larger than a Durable Object value (128 KiB), and replays it to a viewer that arrives while the machine is away. For an encrypted session those bytes are ciphertext it cannot read: the opcode is authenticated as associated data, so the frame is stored and replayed untouched. It is deleted with the session.
• A host only produces a screen when asked, so the alarm asks for one every five minutes on behalf of viewer 0, which no viewer ever is. That caps staleness for the cost of one snapshot per five minutes of a live session. Writes are throttled so sixteen people opening a link does not write sixteen screens.

Both viewers

• Waiting for a machine that has never connected, or temporarily offline with when it was last seen and how old the screen below it is.
• A kept screen is dimmed so it cannot be read as a live one.
• It clears itself when the machine comes back.

Retention note

The relay now holds the last screen of each session, encrypted and unreadable by it, for as long as that session exists (12 hours, or 30 days for a persistent one). For a --no-e2ee session those bytes are plaintext, which is the tradeoff that flag already makes in flight.

Verification

Unit: tests/host-presence.test.ts (9) for the copy and the elapsed-time wording, and four cases in app/src/terminal/connection.test.ts for the status mapping. The connection tests fail on main and pass here.

End to end against a local relay and a real CLI:

1. Live session renders, no notice.
2. Host SIGKILLed — the open viewer keeps its screen, dimmed, under "Temporarily offline… last seen just now".
3. A viewer arriving afterwards gets the kept screen and the notice, where it used to get a blank page.
4. A restarted persistent host clears the notice and the terminal goes live again.
5. A session whose machine never connected shows "Waiting for this machine".

Checked in both themes and in the app pane as well as the standalone page.

No CLI change, so no release is needed; cmd/ and internal/ are untouched.