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/include/common.php

@@ -536,6 +536,41 @@ function vv_array_disks(): array {
     return array_merge($parity, $data);
 }
 
+function vv_disk_io_rates(): array {
+    $snapFile = '/tmp/vv_diskio_snap.json';
+    $now      = microtime(true);
+
+    // Read current whole-disk stats from /proc/diskstats
+    $current = [];
+    foreach (@file('/proc/diskstats', FILE_IGNORE_NEW_LINES) ?: [] as $line) {
+        $p = preg_split('/\s+/', trim($line));
+        if (count($p) < 14) continue;
+        $dev = $p[2];
+        // Keep only whole disks: sda/sdb, nvme0n1, md*, not sda1/nvme0n1p1
+        if (!preg_match('/^(sd[a-z]+|nvme\d+n\d+|md\d+)$/', $dev)) continue;
+        $current[$dev] = [(int)$p[5], (int)$p[9]]; // [sectors_read, sectors_written]
+    }
+
+    // Load previous snapshot
+    $snap     = @json_decode(@file_get_contents($snapFile) ?: '', true) ?: [];
+    $prevTime = (float)($snap['t'] ?? $now);
+    $prev     = $snap['d'] ?? [];
+
+    // Save current snapshot
+    @file_put_contents($snapFile, json_encode(['t' => $now, 'd' => $current], JSON_UNESCAPED_UNICODE));
+
+    $dt    = max(0.5, $now - $prevTime);
+    $rates = [];
+    foreach ($current as $dev => [$rs, $ws]) {
+        if (!isset($prev[$dev])) continue;
+        [$prs, $pws] = $prev[$dev];
+        $r = max(0.0, ($rs - $prs) * 512 / $dt / 1048576);
+        $w = max(0.0, ($ws - $pws) * 512 / $dt / 1048576);
+        if ($r > 0.01 || $w > 0.01) $rates[$dev] = [round($r, 1), round($w, 1)];
+    }
+    return $rates;
+}
+
 function vv_disk_thresholds(): array {
     $cfg  = @file_get_contents('/boot/config/plugins/dynamix/dynamix.cfg') ?: '';
     $get  = function(string $key) use ($cfg): ?int {