General bug fixes and stability improvements

- Improved performance when opening very large libraries for the first time.
- Customize your study sets with any color (now available to everyone).
- Fixed several layout issues in split view and the sidebar, including search not restoring correctly.
- Fixed Whiteboard issues, including presentation mode fallback, folder creation layout, and import text.
- Fixed issues in Text Documents, including missing links in PDF exports, a lingering cursor on Mac while scrolling, and the link popover not closing after adding a link.

- Introducing Text Documents: A block-based editor for fast typing
- Multi tabs: Switch between documents using tabs without losing your place
- Audio Recording: Record audio while taking notes
- Notebook Organisation: Move documents and folders freely, including nested folders
- Introducing Goodnotes Pro: A new subscription for real-time collaboration and full AI access
- Goodnotes AI: Your built-in collaborator to help you think faster, write better, and do more