feat(telemetry): Node Health Server Info panels + ledger-seq descriptions

Sync the Node Health dashboard with panels and field settings refined in
Grafana, and document the ledger-convergence panels:

- Add descriptions for "Validated Ledger Seq — Current (Stat)" (per-node lag
  behind the network tip) and "Validated Ledger Seq — Convergence (Max − Min)"
  (seq spread across nodes), in the standard 7-field panel format.
- Peer Count: threshold colours (red < 5, yellow < 10, green >= 10) so a low
  peer count reads red rather than using a value gradient.
- Minor field-config fixes on Last Close and NuDB Storage panels (axis label,
  threshold cleanup) as adjusted in Grafana.

Add the Grafana duration unit "dthms" to the cspell dictionary. Panel content
stays at parity with the Grafana Cloud copy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.cspell.config.yaml

@@ -108,6 +108,7 @@ words:
   - disablerepo
   - distro
   - doxyfile
+  - dthms
   - dxrpl
   - elgamal
   - enabled