[docs] added tui improvements and stub docs for frontends

[om-raheja]

I added some fair color support to the TUI as well as parser args to the agent_repl. Now it mostly works again. I added stub documentation for the frontend implementation standard. I also deleted the hardcoded gpt4o from server.py and instead set that to the default in parser_args, so that you can pass arguments if you don't want the default gpt4o.

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