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/Controllers/DhcpController.cs

@@ -42,6 +42,10 @@ public class DhcpController : ControllerBase
             var reload = await _reloadService.ReloadAsync(ct);
             return Ok(new SaveWithReloadResult(true, reload));
         }
+        catch (ArgumentException ex)
+        {
+            return BadRequest(new { error = ex.Message });
+        }
         catch (Exception ex)
         {
             return StatusCode(500, new { error = ex.Message });