Effective config: multi descriptor refactor, validation UX, restore cleanup

- All multi fields use EffectiveMultiValueConfigFieldDescriptor; registry has
  RegisterMultiDescriptor/RegisterMultiComponent/RegisterMulti; every multi
  option registered; plain multi fallback removed (AddMultiDescriptor throws).
- Descriptor-driven validation: base descriptor ValidateValue + Validate();
  integer descriptor enforces Min/Max; ListenAddressMultiValidator for
  listen-address; single/flag commit uses Descriptor.Validate (no session
  field issues for inline edits).
- Multi list editor: local _displayItems so remove/add/edit update UI
  immediately; remove button PreventBlurOnMouseDown; inline validation
  message under input (ec-multi-list-input-group); root div not span.
- Save flow: on successful backup restore call OnCloseAndEndEdit then
  OnSaveCompleted so session and pending changes are cleared and config
  is reloaded.
- Server uses descriptor-only (no custom multi component); ServerMultiValueDisplay
  removed. RegisterValidatedSingle added for future single-field validation.

src/DnsmasqWebUI/Components/EffectiveConfig/Modals/EffectiveConfigSaveFlow.razor

@@ -64,6 +64,11 @@
     private async Task HandleSave()
     {
         if (Session.PendingChanges.Count == 0) return;
+        if (Session.HasBlockingValidationErrors())
+        {
+            NotificationService.ShowError("Fix validation errors before saving.");
+            return;
+        }
 
         _saveState = SaveUiState.Saving;
         _saveResult = null;
@@ -114,7 +119,7 @@
             _saveState = SaveUiState.Review;
             _saveResult = null;
             _restoreResult = null;
-            await OnClose.InvokeAsync();
+            await OnCloseAndEndEdit.InvokeAsync();
             await OnSaveCompleted.InvokeAsync();
             return;
         }