fix: open external links in VS Code runtime

Route shared URL opening through VS Code host instead of webview window APIs
Add a VS Code runtime URL-open API and bridge handler using vscode.env.openExternal
Keep shared URL helper behavior unchanged for desktop and web

packages/ui/src/lib/api/types.ts

@@ -619,6 +619,7 @@ export interface EditorAPI {
 export interface VSCodeAPI {
   executeCommand(command: string, ...args: unknown[]): Promise<unknown>;
   openAgentManager(): Promise<void>;
+  openExternalUrl(url: string): Promise<void>;
 }
 
 export interface PushSubscribePayload {