More AI chat providers and smoother iCloud editing

New

• Amp and OpenCode join the AI chat options, with OpenCode models grouped by provider in the existing picker.
• Copy buttons appear on hover for inline code and preview code blocks.

Improved

• iCloud-backed notes stay responsive while typing — your own autosaves no longer register as outside changes.
• Switching models mid-chat keeps the current note's transcript intact instead of fragmenting history per model.
• qmd indexing for related notes works when installed through Bun.

Fixed

• Nested code fences with shorter inner backticks render correctly instead of breaking out of the outer block.
• Markdown files dropped from Finder onto the note surface now open in the editor.
• Inserting or deleting emoji no longer shifts the rest of the source line.