docs(monitor): document join/leave browser notifications

Records why it is browser-only rather than chat push (a push channel needs a CA
bundle in the scratch image, a token, and outbound egress this container does
not have), why newness is a localStorage seen-set keyed on the log timestamp
rather than a server-side sequence number (the counter resets on restart, the
timestamp does not), and the unconditional-baseline subtlety.

Also states plainly what the node harness does NOT cover: the real permission
prompt and actual notification rendering are stubbed, so those are untested.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -169,6 +169,10 @@ ERROR/WARN/INFO chips, above a row of tiles: server status, TPS, players online
 (with NMSR heads), registered accounts, disk free, backup freshness and recent
 joins/leaves.
 
+The bell in the header turns on **browser notifications for player join/leave**.
+Opt-in, off by default, remembered per browser; it fires only while the page is
+open (a background tab counts, a closed one doesn't).
+
 **Admin only, and read-only.** The log carries player IPs, chat and admin
 commands verbatim and nothing sanitizes it, so the whole vhost sits behind caddy
 `basic_auth`. There are no controls in the UI — every route is a `GET`.