[Feature] Implement workspace sharing

As a user, I want to share my workspaces with team members, so that we can collaborate on AI conversations and build knowledge together.

As a team member, I want to invite colleagues to my workspace, so that we can work together on AI-assisted projects in real-time. (Collaboration focus)

As a user, I want to share workspaces with my team, so that we can build a shared knowledge base from our AI interactions. (Knowledge sharing)