Add Cmd+Shift+[ / ] hotkeys for switching between threads

[shawandpartners]

Feature Request

Add keyboard shortcuts for navigating between threads/conversations:

• Cmd+Shift+[ — Previous thread
• Cmd+Shift+] — Next thread

Why

This follows the standard macOS tab-switching convention used in Chrome, Safari, VS Code, iTerm2, and most other tabbed apps. It's muscle memory for most macOS users and would make thread navigation much faster than clicking.

Context

Currently there doesn't appear to be a keyboard shortcut for switching between threads. For power users running multiple conversations, this is a frequent interaction that should be keyboard-accessible.