feat(centralui): compact multi-select dropdowns for the audit filter bar

Replace the four stacked chip-button groups (Channel, Kind, Status, Site) on
the Audit Log filter bar with a reusable MultiSelectDropdown component, so the
bar collapses from four full-width chip blocks to four inline dropdowns sharing
one wrapped filter row. Bootstrap dropdown + checkbox menu (data-bs-auto-close
=outside); no third-party UI libraries.

src/ScadaLink.CentralUI/Components/Shared/MultiSelectDropdown.razor.cs

@@ -0,0 +1,95 @@
+using Microsoft.AspNetCore.Components;
+
+namespace ScadaLink.CentralUI.Components.Shared;
+
+/// <summary>
+/// A compact multi-select control: a Bootstrap dropdown whose toggle button
+/// summarises the current selection ("All" when empty, the single item's label
+/// when one is picked, or "N selected" otherwise) over a checkbox menu.
+///
+/// <para>
+/// It exists to keep multi-value filter controls one row tall instead of a
+/// wrapped block of chip buttons. The component mutates the caller-owned
+/// <see cref="Selected"/> collection in place and raises
+/// <see cref="SelectionChanged"/> after every toggle so the parent can react
+/// (re-render, prune dependent selections, …).
+/// </para>
+///
+/// <para>
+/// Requires the Bootstrap JS bundle (loaded in <c>App.razor</c>) for the
+/// dropdown toggle; <c>data-bs-auto-close="outside"</c> keeps the menu open
+/// while the operator ticks several boxes.
+/// </para>
+/// </summary>
+/// <typeparam name="TValue">The option value type (an enum or string).</typeparam>
+public partial class MultiSelectDropdown<TValue> where TValue : notnull
+{
+    /// <summary>The options shown in the menu, in display order.</summary>
+    [Parameter, EditorRequired]
+    public IReadOnlyList<TValue> Items { get; set; } = Array.Empty<TValue>();
+
+    /// <summary>
+    /// The caller-owned selection set. Mutated in place by <see cref="Toggle"/>.
+    /// </summary>
+    [Parameter, EditorRequired]
+    public ICollection<TValue> Selected { get; set; } = default!;
+
+    /// <summary>Maps an option to its display label. Defaults to <c>ToString()</c>.</summary>
+    [Parameter]
+    public Func<TValue, string> Display { get; set; } = static v => v.ToString() ?? string.Empty;
+
+    /// <summary>Raised after each toggle, once <see cref="Selected"/> has been updated.</summary>
+    [Parameter]
+    public EventCallback SelectionChanged { get; set; }
+
+    /// <summary>Summary text shown on the toggle button when nothing is selected.</summary>
+    [Parameter]
+    public string AllLabel { get; set; } = "All";
+
+    /// <summary>Text shown in the menu when there are no options.</summary>
+    [Parameter]
+    public string EmptyText { get; set; } = "None available";
+
+    /// <summary><c>data-test</c> root for this control, its toggle and its options.</summary>
+    [Parameter]
+    public string DataTest { get; set; } = "multi-select";
+
+    private async Task Toggle(TValue item)
+    {
+        // ICollection.Remove returns false when the item was absent — that is the
+        // "not currently selected" case, so add it. This is a plain toggle.
+        if (!Selected.Remove(item))
+        {
+            Selected.Add(item);
+        }
+
+        await SelectionChanged.InvokeAsync();
+    }
+
+    private string Summary()
+    {
+        var count = Selected.Count;
+        if (count == 0)
+        {
+            return AllLabel;
+        }
+
+        if (count == 1)
+        {
+            // Prefer the single selection's label over a bare "1 selected".
+            foreach (var item in Items)
+            {
+                if (Selected.Contains(item))
+                {
+                    return Display(item);
+                }
+            }
+
+            // The one selected value is not in the current Items list (e.g. a Kind
+            // narrowed out by a Channel change before the parent pruned it).
+            return "1 selected";
+        }
+
+        return $"{count} selected";
+    }
+}