UX: AI composer helper refinements

[keegangeorge]

🔍 Overview

This update includes a variety of small refinements to the AI composer helper:

• prevent height jump when going from loading text placeholder → proofreading text streaming
• update padding on AI helper options list to be more suitable with typical Discourse menu design
• for composer helper results that are not showResultAsDiff (i.e. translation):
  • update before/after diff design to be more subtle
  • results should be in normal font (as the text is cooked and not raw markdown)
• fix: smooth streaming animation stuck showing dot icon even after smooth streaming is done

📷 Screenshots

Before	After