Save threads as drafts

Allow users to save a starting message as a thread draft. This lets a user pause a conversation, open a new thread to process or transform part of the content, then return to the draft and continue seamlessly.

Example workflow:
I was working on a programming project (small enough to fit in context), attached the relevant files, and wrote a prompt to generate a summary for my CV. I realized I also wanted to include the job offer I was applying for — normally I open a new thread in T3 Chat to convert a job offer to Markdown, then copy the result back. Without drafts I can’t pause the main thread, do that transformation, and return cleanly. This simple case illustrates the workflow; more complex uses could benefit more from this feature.