2.18.0-beta.24 — the beta catches up with stable

Everything from 2.17.6 and 2.17.7, carried onto the multi-host line.

The beta had fallen behind :latest — it was built on 27 August and none of
the stable work since had reached it, so anyone testing the beta was running
older software than anyone who wasn't. That is fixed: the precise Compose
warning, the Compose block on the container page, the self-update button and
the updating badge, notifications that survive a network outage, the crash
alert that says when it measured, /changelog reading on the right host,
the parallel host queries and the shortened waits for a dead one, the data
directory that stops claiming /data, and the storage check that no longer
accuses a volume somebody else mounted. The full text for each is in the
2.17.6 and 2.17.7 entries below.

Three of them did not port mechanically and were re-plumbed for this line,
where the self-update lives in its own module rather than inside the bots:
the hint on a denied pull, the private reply route (which now travels
start → run → swap → write_marker and through the queue), and keeping the
"restarting" notice out of the Discord channel when the request was private.

Fixed

• A held notification could be dropped by the flush that was meant to send it. The retry queue had no lock: an entry appended between the end of flush's loop and its closing assignment was discarded — not retried, not reported. A narrow window, and exactly the one an outage produces, since that is when both sides run at once.
• A private self-update silenced unrelated start-up notices. Every boot notice took the private route while a reply address was set, so a storage-misconfiguration warning went to one person's DMs instead of the channel the operator watches. Only the answer to the request does that now.
• A private self-update still narrated itself in the channel. The ephemeral answer said "I'll send you a direct message", and the very next line — 🔄 Checking update for ghcr.io/…, with the image id — went to the channel for everyone. Found by running one: we already keep the restart notice out of there for exactly this reason, and doing one without the other is half a promise. The running commentary now follows the request; a DM that cannot be opened falls back to the channel rather than losing the report.
• A Compose label naming both a manager's file and a host-side override produced an unusable mount line. The exact lookup gave up on the whole set if any one path had no holder, and the fallback then offered to mount a container-internal path onto itself. Each path is answered on its own.