feat: add stats polling interval + correct health polling default to Settings

- Add 'Stats Polling Interval' (default 30s) field to Health -> Configure ->
  Global Settings, alongside the existing 'Polling Interval' which is now
  relabeled 'Health Check Polling Interval' with the correct default of 60s.
- Persist/load statsPollingInterval via the health-settings localStorage key
  (same mechanism as the other settings).
- Wire the persisted statsPollingInterval into DC.POLL.STATS so the resource
  monitor / monitoring widgets honor the configured cadence (seconds -> ms).
- Update the setup wizard disk-safety 'Recommended after setup' list to
  mention both polling intervals.
- Rebuild dist bundles (core.js, features.js) and bump service-worker cache.

status/index.html

@@ -705,6 +705,8 @@
           <strong style="font-size: 0.9rem;">📋 Recommended after setup</strong>
           <ul style="margin: 8px 0 0; padding-left: 20px; font-size: 0.85rem; color: var(--muted); line-height: 1.6;">
             <li>Open <strong>Health → Configure → Global Settings</strong></li>
+            <li>Set a <strong>health check polling interval</strong> (default: 60s)</li>
+            <li>Set a <strong>stats polling interval</strong> (default: 30s)</li>
             <li>Set a <strong>data retention period</strong> (default: 30 days)</li>
             <li>Cap <strong>max entries per service</strong> (default: 500)</li>
             <li>Set a <strong>disk-usage warning threshold</strong> (default: 80%)</li>