feat: add custom context menus for editor and chat input

- Add EditorContextMenu for code editor with clipboard operations
  (Undo, Redo, Cut, Copy, Paste, Select All)
- Add TextInputContextMenu for chat input textarea
- Add global context menu prevention to disable default webview menu
- Add viewport boundary detection to keep menus within window bounds
- Install @codemirror/commands for undo/redo functionality

package.json

@@ -27,6 +27,7 @@
   },
   "license": "MIT",
   "dependencies": {
+    "@codemirror/commands": "6.8.1",
     "@codemirror/lang-angular": "^0.1.4",
     "@codemirror/lang-cpp": "^6.0.3",
     "@codemirror/lang-css": "^6.3.1",