refactor(ui-common): generic WebSocket adapter factory with converter injection

refactor(ui-common): generic WebSocket adapter factory with converter injection

Create ui/common/src/client/adapter.ts — a generic createWsAdapter(ws, options)
factory that implements all WebSocketLike callback boilerplate once. Both adapters
become thin wrappers specifying only their platform-specific data converter:

- createBrowserWsAdapter: data => data as string, errorDefault: 'WebSocket error'
- createWsAdapter (CLI): toDataString() for Buffer/ArrayBuffer, errorDefault: 'Unknown error'

Net reduction: ~143 lines of duplicated boilerplate eliminated.
Add DataConverter type and tests for the generic factory.