Document all available keyboard shortcuts in README

[yandanp]

Description

The app has keyboard shortcuts but they're not documented anywhere users can easily find them.

Task

• Explore the app and identify all keyboard shortcuts
• Add a "Keyboard Shortcuts" section to README.md (or a separate SHORTCUTS.md)
• Group by category (terminal, tabs, navigation, etc.)

Tech

• Documentation only — no code changes needed
• Check src/renderer/ for keybinding definitions

Getting Started

See CONTRIBUTING.md for setup instructions.