feat: add embed-pdf package and refactor state management

[ibastawisi]

This pull request introduces several improvements and refactors to the web application, focusing on state management organization, enhanced document navigation, PDF viewing, and better support for mobile and virtual keyboards. The most significant changes are grouped below.

State Management Refactor:

• Refactored Redux state selectors throughout the codebase to use namespaced paths (e.g., state.wordy.* and state.ui.*) instead of flat state properties. This improves maintainability and clarity of state usage. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Document Navigation and Context Menu Enhancements:

• Added "Open in new tab" and "Open to the side" options to the document context menu, including keyboard shortcuts and support for opening in a split pane based on orientation.
• Improved document context menu UI and logic, including new icons and shortcut displays. [1] [2] [3] [4]

PDF Viewing Improvements:

• Integrated a new @repo/embed-pdf package and replaced the native <iframe> with a custom PDFViewer component for displaying PDFs in attachments. [1] [2] [3]

Mobile and Virtual Keyboard Support:

• Improved mobile and virtual keyboard handling by adding support for the geometrychange event and updating the CSS to better handle contenteditable elements and context menus on touch devices. [1] [2] [3]

UI and Miscellaneous Enhancements:

• Minor UI improvements, such as conditional icon coloring in the space context menu and code cleanup in the split pane router and other components. [1] [2] [3] [4]

These changes collectively improve the maintainability, usability, and user experience of the application.