feat(centralui): single-select Channel filter on the Audit Log page

Channel narrows the Kind options to the chosen channel, so filtering by more
than one channel at a time is incoherent. Replace the Channel multi-select
dropdown with a native single-select (matching the Time range control); Kind,
Status and Site stay multi-select. The query filter contract is unchanged —
Channels just carries 0 or 1 value.

src/ScadaLink.CentralUI/Components/Audit/AuditFilterBar.razor

@@ -6,20 +6,23 @@
 
 <div class="card mb-3" data-test="audit-filter-bar">
     <div class="card-body py-2">
-        @* All filters sit in one wrapped row. The four multi-value dimensions
+        @* All filters sit in one wrapped row. Kind / Status / Site use compact
-           (Channel / Kind / Status / Site) use compact MultiSelectDropdown
+           MultiSelectDropdown controls; Channel is a single-select because the
-           controls so the bar stays a row or two tall instead of four stacked
+           Kind options narrow to the chosen channel — so the bar stays a row or
-           blocks of chip buttons. *@
+           two tall instead of four stacked blocks of chip buttons. *@
         <div class="row g-2 align-items-end">
+            @* Single-select: one channel at a time, so the Kind options below
+               narrow cleanly to that channel. "All channels" clears it. *@
             <div class="col-auto" data-test="filter-channel">
-                <label class="form-label small mb-1">Channel</label>
+                <label class="form-label small mb-1" for="audit-channel">Channel</label>
-                <div>
+                <select id="audit-channel" data-test="filter-channel-select"
-                    <MultiSelectDropdown TValue="AuditChannel"
+                        class="form-select form-select-sm" @bind="SelectedChannel">
-                                         Items="_channels"
+                    <option value="">All channels</option>
-                                         Selected="_model.Channels"
+                    @foreach (var channel in _channels)
-                                         SelectionChanged="OnChannelsChanged"
+                    {
-                                         DataTest="filter-channel-ms" />
+                        <option value="@channel">@channel</option>
-                </div>
+                    }
+                </select>
             </div>
 
             @* Kind options are narrowed by the Channel selection (VisibleKinds). *@

src/ScadaLink.CentralUI/Components/Audit/AuditFilterBar.razor.cs

@@ -8,13 +8,14 @@ namespace ScadaLink.CentralUI.Components.Audit;
 /// <summary>
 /// Filter bar for the central Audit Log page (#23 M7-T2). Owns the
 /// <see cref="AuditQueryModel"/> binding state and renders the filter controls
-/// — Channel / Kind / Status / Site as compact
+/// — Channel as a single-select (one channel at a time, so the Kind options
+/// narrow to it cleanly); Kind / Status / Site as compact
 /// <see cref="ScadaLink.CentralUI.Components.Shared.MultiSelectDropdown{TValue}"/>
-/// controls, plus the time range, free-text searches and the Errors-only
+/// controls; plus the time range, free-text searches and the Errors-only
 /// toggle — and publishes an <see cref="AuditLogQueryFilter"/> via
-/// <see cref="OnFilterChanged"/> when the user clicks Apply. The four
+/// <see cref="OnFilterChanged"/> when the user clicks Apply. The selected
-/// multi-value dimensions map straight through to the filter's list fields;
+/// dimensions map through to the filter's list fields; see
-/// see <see cref="AuditQueryModel"/> for the Errors-only and time-range rules.
+/// <see cref="AuditQueryModel"/> for the Errors-only and time-range rules.
 /// </summary>
 public partial class AuditFilterBar
 {
@@ -82,8 +83,29 @@ public partial class AuditFilterBar
     }
 
     /// <summary>
-    /// Runs after a Channel selection changes. Drops any Kind selections that fell
+    /// Single-select Channel binding for the filter bar. The Audit Log filters one
-    /// outside the new visible set — without this, removing a channel could leave
+    /// channel at a time so the Kind options narrow cleanly to it; the model still
+    /// stores the selection as a set (0 or 1 entry) so <see cref="AuditQueryModel.ToFilter"/>
+    /// and <see cref="AuditQueryModel.VisibleKinds"/> are unchanged. <c>null</c> = all channels.
+    /// </summary>
+    private AuditChannel? SelectedChannel
+    {
+        get => _model.Channels.Count > 0 ? _model.Channels.First() : null;
+        set
+        {
+            _model.Channels.Clear();
+            if (value is { } channel)
+            {
+                _model.Channels.Add(channel);
+            }
+
+            OnChannelsChanged();
+        }
+    }
+
+    /// <summary>
+    /// Runs after the Channel selection changes. Drops any Kind selections that fell
+    /// outside the new visible set — without this, changing the channel could leave
     /// stale Kind selections that no longer match any visible option.
     /// </summary>
     private void OnChannelsChanged()

tests/ScadaLink.CentralUI.PlaywrightTests/Audit/AuditLogPageTests.cs

@@ -91,9 +91,8 @@ public class AuditLogPageTests
             await page.WaitForLoadStateAsync(LoadState.NetworkIdle);
 
             // Pre-Apply, both rows are absent because the grid stays empty until
-            // the user filters. Open the Channel dropdown, tick ApiOutbound, Apply.
+            // the user filters. Pick the ApiOutbound channel, then Apply.
-            await page.Locator("[data-test='filter-channel-ms-toggle']").ClickAsync();
+            await page.Locator("[data-test='filter-channel-select']").SelectOptionAsync("ApiOutbound");
-            await page.Locator("[data-test='filter-channel-ms-opt-ApiOutbound']").ClickAsync();
             await page.Locator("[data-test='filter-apply']").ClickAsync();
             await page.WaitForLoadStateAsync(LoadState.NetworkIdle);
 

tests/ScadaLink.CentralUI.Tests/Components/Audit/AuditFilterBarTests.cs

@@ -13,10 +13,11 @@ namespace ScadaLink.CentralUI.Tests.Components.Audit;
 /// <summary>
 /// bUnit tests for <see cref="AuditFilterBar"/> (#23 M7-T2 / Bundle B).
 ///
-/// The bar carries the 10 spec filter elements plus the Errors-only toggle. The
+/// The bar carries the 10 spec filter elements plus the Errors-only toggle.
-/// Channel / Kind / Status / Site dimensions are rendered as
+/// Channel is a single-select <c>&lt;select data-test="filter-channel-select"&gt;</c>;
+/// Kind / Status / Site are
 /// <see cref="ScadaLink.CentralUI.Components.Shared.MultiSelectDropdown{TValue}"/>
-/// controls; each option is a checkbox tagged
+/// controls whose options are checkboxes tagged
 /// <c>data-test="filter-&lt;dim&gt;-ms-opt-&lt;value&gt;"</c>. Tests pin:
 /// (1) the full filter set renders; (2) Apply raises <c>OnFilterChanged</c> with
 /// the selected values; (3) the Channel→Kind narrowing map drives Kind option
@@ -75,8 +76,8 @@ public class AuditFilterBarTests : BunitContext
         var cut = Render<AuditFilterBar>(p => p
             .Add(c => c.OnFilterChanged, EventCallback.Factory.Create<AuditLogQueryFilter>(this, f => captured = f)));
 
-        // Drive UI: tick a Channel option, type in the Target search box, click Apply.
+        // Drive UI: pick a Channel, type in the Target search box, click Apply.
-        cut.Find("[data-test=\"filter-channel-ms-opt-ApiOutbound\"]").Change(true);
+        cut.Find("[data-test=\"filter-channel-select\"]").Change("ApiOutbound");
         cut.Find("[data-test=\"filter-target\"] input").Change("Plant-A-OPC");
         cut.Find("[data-test=\"filter-apply\"]").Click();
 
@@ -86,23 +87,25 @@ public class AuditFilterBarTests : BunitContext
     }
 
     [Fact]
-    public void Apply_WithMultipleChannelChips_PassesAllSelectedChannels()
+    public void ChangingChannel_ReplacesTheSelection_SingleSelect()
     {
-        // Task 9: ToFilter no longer collapses the chip multi-select — every
+        // Channel is single-select: picking a second channel replaces the first
-        // selected channel chip reaches the filter's Channels list.
+        // rather than adding to it (the page filters one channel at a time).
         AuditLogQueryFilter? captured = null;
         var cut = Render<AuditFilterBar>(p => p
             .Add(c => c.OnFilterChanged, EventCallback.Factory.Create<AuditLogQueryFilter>(this, f => captured = f)));
 
-        cut.Find("[data-test=\"filter-channel-ms-opt-ApiOutbound\"]").Change(true);
+        cut.Find("[data-test=\"filter-channel-select\"]").Change("ApiOutbound");
-        cut.Find("[data-test=\"filter-channel-ms-opt-Notification\"]").Change(true);
+        cut.Find("[data-test=\"filter-channel-select\"]").Change("Notification");
         cut.Find("[data-test=\"filter-apply\"]").Click();
 
         Assert.NotNull(captured);
-        Assert.NotNull(captured!.Channels);
+        Assert.Equal(new[] { AuditChannel.Notification }, captured!.Channels);
-        Assert.Equal(2, captured.Channels!.Count);
+
-        Assert.Contains(AuditChannel.ApiOutbound, captured.Channels);
+        // Selecting "All channels" clears the channel filter entirely.
-        Assert.Contains(AuditChannel.Notification, captured.Channels);
+        cut.Find("[data-test=\"filter-channel-select\"]").Change(string.Empty);
+        cut.Find("[data-test=\"filter-apply\"]").Click();
+        Assert.Null(captured!.Channels);
     }
 
     [Fact]
@@ -116,8 +119,8 @@ public class AuditFilterBarTests : BunitContext
             Assert.Contains($"data-test=\"filter-kind-ms-opt-{kind}\"", cut.Markup);
         }
 
-        // Select only ApiOutbound; Kind options outside the channel-kind map drop out.
+        // Select ApiOutbound; Kind options outside the channel-kind map drop out.
-        cut.Find("[data-test=\"filter-channel-ms-opt-ApiOutbound\"]").Change(true);
+        cut.Find("[data-test=\"filter-channel-select\"]").Change("ApiOutbound");
 
         var apiKinds = AuditQueryModel.KindsByChannel[AuditChannel.ApiOutbound];
         foreach (var kind in apiKinds)