Cross-option validation: field keys, bogus-priv zones, edit UX

- Add FieldKeyForOption() using EffectiveConfigSections.GetSectionId so
  cross-option issues match UI field keys; tighten RFC1918 in-addr.arpa
  detection for bogus-priv vs server=.
- GetMulti: pending change always wins; empty list when NewValue is not
  a string list (with tests).
- EffectiveConfigSection: re-run cross-option when entering edit mode or
  continuing after save failure; allow opening save modal when only
  validation summary is present.
- Toolbar: validation link aria-label describes opening the summary.

src/DnsmasqWebUI/Components/EffectiveConfig/Composition/EffectiveConfigSection.razor

@@ -155,6 +155,7 @@
     private void EnterEditMode()
     {
         Session.EnterEditMode();
+        RefreshCrossOptionAfterEnteringEditMode();
         StateHasChanged();
     }
 
@@ -206,6 +207,15 @@
         Session.SetCrossOptionIssues(issues);
     }
 
+    /// <summary>
+    /// <see cref="IEffectiveConfigEditSession.EnterEditMode"/> clears cross-option issues; repopulate from disk + pending.
+    /// </summary>
+    private void RefreshCrossOptionAfterEnteringEditMode()
+    {
+        if (Status != null)
+            RunCrossOptionEvaluator();
+    }
+
     private void OnRevertedFromSaveModal()
     {
         RunCrossOptionEvaluator();
@@ -214,7 +224,10 @@
 
     private void OpenSaveModal()
     {
-        if (Session.PendingChanges.Count == 0) return;
+        var hasPending = Session.PendingChanges.Count > 0;
+        var hasValidation = Session.GetValidationSummary().Count > 0;
+        if (!hasPending && !hasValidation)
+            return;
         _showSaveModal = true;
         StateHasChanged();
     }
@@ -231,6 +244,7 @@
         Session.ExitEditModeDiscard();
         await OnSaveCompleted.InvokeAsync();
         Session.EnterEditMode();
+        RefreshCrossOptionAfterEnteringEditMode();
         StateHasChanged();
     }
 

src/DnsmasqWebUI/Components/EffectiveConfig/Composition/EffectiveConfigToolbar.razor

@@ -37,7 +37,7 @@
                     {
                         <span class="text-danger fw-medium ec-toolbar-changes-link" role="button" tabindex="0"
                               title="@ValidationErrorTooltip"
-                              aria-label="Open save dialog"
+                              aria-label="Open validation summary"
                               @onclick="OnOpenSaveModal"
                               @onkeydown="OnValidationLinkKeyDown">
                             @ValidationErrorCount error(s)
@@ -48,7 +48,7 @@
                     {
                         <span class="text-warning ec-toolbar-changes-link" role="button" tabindex="0"
                               title="@ValidationWarningTooltip"
-                              aria-label="Open save dialog"
+                              aria-label="Open validation summary"
                               @onclick="OnOpenSaveModal"
                               @onkeydown="OnValidationLinkKeyDown">
                             @ValidationWarningCount warning(s)