feat(adminui): T0-1 reusable ContextMenu component + demo page

Adds a reusable right-click / context menu Blazor component for the AdminUI:
- ContextMenu.razor: right-click surface (browser menu suppressed) + optional
  ellipsis fallback trigger, both anchoring the menu at the pointer; keyboard
  nav (arrows/Enter/Esc), transparent backdrop outside-click close (no JS).
- ContextMenu.razor.css: scoped styles on the shared theme tokens.
- ContextMenuItem.cs: Label/OnClick/Icon/Disabled/IsSeparator model + Separator().
- Dev demo page at /dev/context-menu-demo.

Claude-Session: https://claude.ai/code/session_01LVneM3eh1UtJxEisFXgmox

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Pages/Dev/ContextMenuDemo.razor

@@ -0,0 +1,35 @@
+@page "/dev/context-menu-demo"
+@* Dev-only host page to live-verify the reusable ContextMenu component. *@
+@attribute [Authorize(Policy = AdminUiPolicies.AuthenticatedRead)]
+@rendermode InteractiveServer
+
+<PageTitle>ContextMenu demo</PageTitle>
+
+<h1>ContextMenu demo</h1>
+<p>Right-click the card below, or use the <strong>⋯</strong> button.</p>
+
+<div style="display:flex; align-items:center; gap:1rem; margin:1rem 0;">
+    <ContextMenu Items="_items" ShowEllipsis="false">
+        <div style="padding:2rem 3rem; border:1px dashed var(--bs-border-color); border-radius:.5rem; user-select:none;">
+            Right-click me
+        </div>
+    </ContextMenu>
+
+    <ContextMenu Items="_items" ShowEllipsis="true" />
+</div>
+
+<p>Last action: <strong>@_lastAction</strong></p>
+
+@code {
+    private string _lastAction = "(none)";
+
+    private IReadOnlyList<ContextMenuItem> _items => new List<ContextMenuItem>
+    {
+        new() { Label = "Edit", Icon = "✏️", OnClick = () => Set("Edit") },
+        new() { Label = "Duplicate", Icon = "📄", OnClick = () => Set("Duplicate") },
+        ContextMenuItem.Separator(),
+        new() { Label = "Delete", Icon = "🗑️", Disabled = true, OnClick = () => Set("Delete") },
+    };
+
+    private void Set(string action) => _lastAction = action;
+}