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>

CLAUDE.md

@@ -251,7 +251,8 @@ Full design settled across WS1–WS6; all tasks unchecked, ready to execute.
   **outside mc-backup** and unquota'd (accepted — eyeballed).
 - [x] **Monitor** (`monitor.`, 2026-08-01) — admin-only `latest.log` stream +
   dashboard tiles (status, TPS via allowlisted RCON, players, disk, backups,
-  joins/leaves). Live on cochi, all three phases done. See `plan/23-monitor.md`.
+  joins/leaves) + opt-in browser notifications on join/leave (bell toggle,
+  client-side only). Live on cochi, all four phases done. See `plan/23-monitor.md`.
   Follow-up: add the Uptime Kuma monitor for `http://monitor:8080/healthz`.
 - [ ] **Verify NeoForge version** against current stable before first deploy.
 - [x] **Bootstrap Drasl admin** account (`admin`, 2026-06-10). Key is