FE: Support selected text in WebUI

When sending a user prompt to GrapQl mutation, we should also send the currently selected text on the screen. This is to provide the backend with additional context when picking a tool and responding to user's prompts. The simplest scenario for this is:

• User selects some text on the screen
• opens the chat and asks something like: "explain this"

Without sending the selected text, the tooling will consider the current page and will, most probably, return the wrong result.