Keyboard shortcuts for sidebar toggling

New shortcuts for a cleaner experience:

• Cmd/Ctrl+Shift+B to toggle the left sidebar;
• z to enter zen mode and hide both sidebars at once.

Other improvements

• AI is now better at understanding which block(s) you're referring to.
• AI is now better at understanding context from past conversation.
• AI prefers simpler maths syntax in custom formulas: + instead of plus(), > over gt() etc.