Smart Apply: Add basic UI to support other IDEs

[umpox]

Description

The current smart apply implementation uses an OS-native dropdown for "Insert at Cursor" and "Save to New File". This is via a VS Code API. It works well for this initial MVP version, but obviously will not work in other clients.

This PR implements a fallback UI for other clients that is the same as the current UI for these actions.

See screenshots

VS Code WebView

Other Clients WebView

Test plan

1. Test all code block actions in VS Code mode in the webview, check they all works
2. Test all code block actions in non-VS Code mode in the webview, check they all works