Detect unsupported quota filesystems (ZFS) and hide quota UI

- _quota_supported() checks df -T /srv; returns false + reason for ZFS,
  tmpfs, overlay — surfaces as quota_supported/quota_reason in dashboard API
- Dashboard quota section shows the reason string instead of an empty table
- Quota button hidden from share rows when quota_supported is false
- Prevents setquota errors for ZFS-backed Proxmox LXC containers

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

static/js/dashboard.js

@@ -36,8 +36,14 @@ async function loadDashboard() {
             </tr>`).join('');
     }
 
+    // Persist quota support flag for shares.js to read
+    window._quotaSupported = d.quota_supported !== false;
+    window._quotaReason = d.quota_reason || '';
+
     const qb = document.getElementById('dashQuotasBody');
-    if (!d.quotas?.length) {
+    if (!d.quota_supported) {
+        qb.innerHTML = `<tr><td colspan="4" style="color:var(--muted); font-size:.87rem">${esc(d.quota_reason)}</td></tr>`;
+    } else if (!d.quotas?.length) {
         qb.innerHTML = `<tr><td colspan="4" style="color:var(--muted)">No quotas set.</td></tr>`;
     } else {
         qb.innerHTML = d.quotas.map(q => {