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>

app.py

@@ -613,6 +613,20 @@ def _shares_with_usage():
     return result
 
 
+def _quota_supported():
+    """Return (supported: bool, reason: str). Checks filesystem type under /srv."""
+    r = subprocess.run(['df', '-T', '/srv'], capture_output=True, text=True)
+    for line in r.stdout.splitlines()[1:]:
+        parts = line.split()
+        if len(parts) >= 2:
+            fstype = parts[1].lower()
+            if fstype == 'zfs':
+                return False, 'ZFS storage detected — Linux disk quotas are not available inside a ZFS-backed container. Manage per-dataset quotas from the Proxmox host with: zfs set quota=10G <dataset>'
+            if fstype in ('tmpfs', 'overlay', 'aufs'):
+                return False, f'{fstype} filesystem does not support disk quotas'
+    return True, ''
+
+
 def _quota_usage():
     try:
         r = subprocess.run(['repquota', '-u', '/srv'], capture_output=True, text=True, timeout=10)
@@ -649,11 +663,14 @@ def _quota_usage():
 @app.route('/api/dashboard', methods=['GET'])
 @require_auth
 def get_dashboard():
+    quota_ok, quota_reason = _quota_supported()
     return jsonify({
         'status': _daemon_status(),
         'disk': _disk_usage(),
         'shares': _shares_with_usage(),
-        'quotas': _quota_usage(),
+        'quotas': _quota_usage() if quota_ok else [],
+        'quota_supported': quota_ok,
+        'quota_reason': quota_reason,
     })