Editor and AI Chat fixes

Improved

• Smoother typing in long notes so editing stays fluid as a note grows.
• Editor layout tweaks with better side margins.

Fixed

• The AI Chat no longer falls back to a Codex model it can't actually use.
• Long notes scroll all the way to the end on macOS Tahoe.
• Toggling preview on a large note stays responsive.
• Inline LaTeX now sits on the same baseline as the text around it.