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/Shared/ContextMenu.razor.css

@@ -0,0 +1,93 @@
+/* Scoped styles for ContextMenu. Colours come from the shared ZB.MOM.WW.Theme
+   tokens (theme.css) so the menu tracks the app palette in light and dark. */
+
+.ctx-surface {
+    display: inline;
+}
+
+/* The "⋯" fallback trigger — a compact, quiet icon button. */
+.ctx-ellipsis {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    width: 1.75rem;
+    height: 1.75rem;
+    padding: 0;
+    border: 1px solid transparent;
+    border-radius: 0.375rem;
+    background: transparent;
+    color: var(--ink-soft);
+    font-size: 1.1rem;
+    line-height: 1;
+    cursor: pointer;
+}
+
+    .ctx-ellipsis:hover,
+    .ctx-ellipsis[aria-expanded="true"] {
+        background: var(--hover-bg);
+        border-color: var(--bs-border-color);
+        color: var(--ink);
+    }
+
+/* Transparent full-viewport catch-all; sits just under the menu. */
+.ctx-backdrop {
+    position: fixed;
+    inset: 0;
+    z-index: 1080;
+    background: transparent;
+}
+
+.ctx-menu {
+    position: fixed;
+    z-index: 1081; /* above the backdrop, below toasts (1090) */
+    min-width: 11rem;
+    max-width: 20rem;
+    padding: 0.25rem;
+    background: var(--card);
+    border: 1px solid var(--bs-border-color);
+    border-radius: 0.5rem;
+    box-shadow: 0 6px 24px rgba(0, 0, 0, 0.16);
+    outline: none;
+}
+
+.ctx-item {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+    width: 100%;
+    padding: 0.35rem 0.6rem;
+    border: 0;
+    border-radius: 0.375rem;
+    background: transparent;
+    color: var(--ink);
+    font-size: 0.875rem;
+    text-align: left;
+    cursor: pointer;
+}
+
+    .ctx-item:hover:not(:disabled),
+    .ctx-item.active:not(:disabled) {
+        background: var(--hover-bg);
+    }
+
+    .ctx-item:disabled {
+        color: var(--ink-faint);
+        cursor: not-allowed;
+    }
+
+.ctx-icon {
+    display: inline-flex;
+    flex: 0 0 auto;
+    width: 1.1rem;
+    justify-content: center;
+}
+
+.ctx-label {
+    flex: 1 1 auto;
+}
+
+.ctx-separator {
+    height: 1px;
+    margin: 0.25rem 0.3rem;
+    background: var(--bs-border-color);
+}