DHCP: read-only hosts from other configs, combobox inputs, polling, UI fixes

- Static hosts: read dhcp-host from all config files; show external as read-only
  with Source column; IsEditable + SourcePath on DhcpHostEntry; reject save if
  managed entry duplicates MAC from another file (400 + message)
- Lease field: single input + datalist (type or select), remove Other dropdown
- MAC field: single input + datalist from current leases, TryFillFromLeaseByMac
- Add host: default Name=*, suggest next free IP from DHCP range
- Leases: configurable refresh polling; Settings modal for interval (client-side)
- DhcpHostsClient: surface 400 error body message on save failure
- App title config (ApplicationOptions), nav/layout/error/hosts tweaks

src/DnsmasqWebUI/Components/Modals/SettingsModal.razor

@@ -38,6 +38,15 @@
                                @bind="_editingSettings.RecentLogsPollingIntervalSeconds" />
                     </section>
                 }
+                @if (ShouldShowSection("Leases"))
+                {
+                    <section class="mb-4">
+                        <h6 class="text-muted">DHCP leases</h6>
+                        <label for="leases-interval" class="form-label">Refresh interval (seconds)</label>
+                        <input id="leases-interval" type="number" class="form-control" min="5" step="1"
+                               @bind="_editingSettings.LeasesPollingIntervalSeconds" />
+                    </section>
+                }
             </div>
             <div class="modal-footer">
                 <button type="button" class="btn btn-secondary" @onclick="Close">Cancel</button>

src/DnsmasqWebUI/Components/Modals/SettingsModal.razor.cs

@@ -42,7 +42,8 @@ public partial class SettingsModal : IAsyncDisposable
             _editingSettings = new ClientSettings
             {
                 ServiceStatusPollingIntervalSeconds = _editingSettings.ServiceStatusPollingIntervalSeconds,
-                RecentLogsPollingIntervalSeconds = _editingSettings.RecentLogsPollingIntervalSeconds
+                RecentLogsPollingIntervalSeconds = _editingSettings.RecentLogsPollingIntervalSeconds,
+                LeasesPollingIntervalSeconds = _editingSettings.LeasesPollingIntervalSeconds
             };
         }
     }
@@ -73,6 +74,8 @@ public partial class SettingsModal : IAsyncDisposable
             return key == "ServiceStatus";
         if (SettingsContext == SettingsModalContext.LogsPolling)
             return key == "Logs";
+        if (SettingsContext == SettingsModalContext.LeasesPolling)
+            return key == "Leases";
         if (SettingsContext == SettingsModalContext.All)
         {
             if (string.IsNullOrWhiteSpace(_searchTerm)) return true;
@@ -81,6 +84,7 @@ public partial class SettingsModal : IAsyncDisposable
             {
                 "ServiceStatus" => "service status polling".Contains(term, StringComparison.OrdinalIgnoreCase),
                 "Logs" => "recent logs polling".Contains(term, StringComparison.OrdinalIgnoreCase),
+                "Leases" => "dhcp leases refresh polling".Contains(term, StringComparison.OrdinalIgnoreCase),
                 _ => false
             };
         }
@@ -93,6 +97,8 @@ public partial class SettingsModal : IAsyncDisposable
             _editingSettings.ServiceStatusPollingIntervalSeconds, 5, 300);
         _editingSettings.RecentLogsPollingIntervalSeconds = Math.Clamp(
             _editingSettings.RecentLogsPollingIntervalSeconds, 5, 300);
+        _editingSettings.LeasesPollingIntervalSeconds = Math.Clamp(
+            _editingSettings.LeasesPollingIntervalSeconds, 5, 300);
 
         await ClientSettingsService.SaveSettingsAsync(_editingSettings);
         if (_jsModule != null)