feat: support empty user prompt with composer input text

[jeanpaulsio]

Summary

• Adds a third Composer input field in the editor panel.
• When the user prompt is left empty, the top user chat block is hidden and the composer renders the typed text before the caret.
• Enables a "Claude said this, here's what I was about to type" framing alongside the existing prompt → response flow.

Test plan

• Default state: user prompt + Claude response + idle caret render unchanged
• Empty user prompt + composer text: user block hidden, composer shows ❯ <text> ▮
• Both filled: user block, Claude response, and composer-with-text all render
• Light + dark themes: composer text uses correct terminal text color
• PNG export still works for each scenario

🤖 Generated with Claude Code