feat: live disk I/O rates on array and pools cards

Adds vv_disk_io_rates() — snapshots /proc/diskstats each poll and
computes per-device MB/s. Exposed as disk_io in the monitor API.

JS:
- Per-disk rows: ↓read ↑write inline next to disk name (green/amber, hidden when idle)
- Pool rows: aggregate pool I/O next to pool name
- Array header: total array I/O in the subtitle line

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Plugin/unraid/api/monitor.php

@@ -25,6 +25,7 @@ echo json_encode([
     'parity'      => vv_parity_status(),
     'storage'     => vv_storage_pools(),
     'array_disks' => vv_array_disks(),
+    'disk_io'     => vv_disk_io_rates(),
     'watchdog'       => vv_watchdog_summary(),
     'scripts'        => vv_scripts_status(),
     'thresholds'     => vv_disk_thresholds(),