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/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'dashcaddy-shell-d17fb082db';
+const CACHE = 'dashcaddy-shell-f0ebe6ce4f';
 const PRECACHE = [
   '/',
   '/index.html',