fix(central-ui): keep the instance picker disabled until the site's instances have loaded (typing raced the load and was wiped)

Both Flow Debugger and Debug View enable the shared InstancePicker as soon as
_selectedSiteId lands, but Blazor renders at the site-change handler's first
await — at which point the picker is still holding the PREVIOUS site's Areas /
Instances lists. Typing into the field in that window (an operator, or
Playwright's FillAsync, which waits only for "enabled") was silently lost: the
picker compares both lists by reference, so the freshly-assigned lists read as
"the site changed" and it clears the field text and closes the panel.

Both pages now hold an _instancesLoading flag across the whole load — set before
the first await, cleared in a finally so a fault or a site-scope refusal cannot
leave the picker disabled forever — and fold it into the picker's Disabled
expression. InstancePicker itself is unchanged.

Claude-Session: https://claude.ai/code/session_01NcMG6xXdsJbmD2wYAgkK29

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Deployment/FlowDebuggerPageTests.cs

@@ -816,6 +816,36 @@ public class FlowDebuggerPageTests : BunitContext
             Assert.Equal("Sim01", cut.Find("[data-test='flow-instance-input']").GetAttribute("value")));
     }
 
+    /// <summary>
+    /// The picker must stay disabled for the WHOLE site load, not just until the site id lands.
+    /// Blazor renders as soon as <c>OnSiteChangedAsync</c> hits its first await, and at that point
+    /// <c>_selectedSiteId</c> is already non-zero while <c>_instances</c>/<c>_areas</c> are still the
+    /// previous site's lists — so an operator (or Playwright's <c>FillAsync</c>, which waits only for
+    /// "enabled") could type into the field and have the load wipe the text and close the panel a
+    /// moment later, because the picker reads a new list object as "the site changed".
+    /// </summary>
+    [Fact]
+    public void ScriptEntry_PickerStaysDisabled_UntilTheSitesInstancesHaveLoaded()
+    {
+        RegisterServices();
+        WireInstance();
+        // Hold the instance load open so the render that follows the first await is observable.
+        var gate = new TaskCompletionSource<IReadOnlyList<Instance>>();
+        _templates.GetInstancesBySiteIdAsync(1, Arg.Any<CancellationToken>()).Returns(gate.Task);
+
+        var cut = Render<FlowDebuggerPage>();
+        cut.Find("button[data-test='flow-entry-script']").Click();
+        cut.Find("select[data-test='flow-site-select']").Change("1");
+
+        Assert.True(cut.Find("[data-test='flow-instance-input']").HasAttribute("disabled"));
+        Assert.True(cut.Find("[data-test='flow-instance-toggle']").HasAttribute("disabled"));
+
+        cut.InvokeAsync(() => gate.SetResult([new Instance("Sim01") { Id = 7, SiteId = 1, TemplateId = 10 }]));
+
+        cut.WaitForAssertion(() =>
+            Assert.False(cut.Find("[data-test='flow-instance-input']").HasAttribute("disabled")));
+    }
+
     [Fact]
     public void ScriptEntry_SiteChange_ClearsThePickedInstance()
     {