feat: global right-click control + topic tree context menu

- UseContextMenu composable: global right-click prevention
  - PROD: prevent default browser context menu everywhere
  - DEV: allow native context menu for debugging
- ContextMenu component: reusable custom right-click menu
- TopicTree: right-click nodes for copy topic / subscribe / unsubscribe / filter
  - Copy topic to clipboard
  - Subscribe with QoS 0/1/2
  - Unsubscribe with confirmation
  - Filter by topic

src/composables/useContextMenu.ts

@@ -0,0 +1,41 @@
+import { onMounted, onUnmounted } from 'vue'
+
+const contextMenuHandlers = new Set<(e: MouseEvent) => void>()
+
+/**
+ * Register a custom right-click handler.
+ * The handler should call `e.preventDefault()` if it handles the event.
+ */
+export function registerContextMenu(handler: (e: MouseEvent) => void) {
+  contextMenuHandlers.add(handler)
+}
+
+export function unregisterContextMenu(handler: (e: MouseEvent) => void) {
+  contextMenuHandlers.delete(handler)
+}
+
+/**
+ * Global context menu controller.
+ * In production: prevents default browser context menu everywhere.
+ * In debug (dev mode): allows native context menu.
+ * Custom handlers registered via registerContextMenu take priority.
+ */
+export function useContextMenuController() {
+  function onContextMenu(e: MouseEvent) {
+    for (const handler of contextMenuHandlers) {
+      handler(e)
+      if (e.defaultPrevented) return
+    }
+    if (import.meta.env.PROD) {
+      e.preventDefault()
+    }
+  }
+
+  onMounted(() => {
+    document.addEventListener('contextmenu', onContextMenu)
+  })
+
+  onUnmounted(() => {
+    document.removeEventListener('contextmenu', onContextMenu)
+  })
+}