Move add folder from a menu item to a button on the project

[ghinkle]

Description

Simple change to move the "Add Folder" menu item in the project tree view to being an icon button on the project to the right of new session.

Just trying to reduce visual clutter when you have many projects open

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the CONTRIBUTING.md guidelines
• My code follows the code style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have run pnpm typecheck and pnpm lint locally
• I have tested the Electron app locally with pnpm electron-dev

Critical Areas Modified

• Session output handling (requires explicit permission)
• Timestamp handling
• State management/IPC events
• Diff viewer CSS

Screenshots (if applicable)

Additional Notes

[jordan-BAIC]

Placement is fine but it does not match the other visuals. Does it work with the light mode/dark mode?