feat: support local visualization.

[IRONICBo]

Summary

• Add a new mem ui command that launches a local web interface for visualizing .mem repository data
• Implement a lightweight web server using Starlette/Uvicorn to serve the UI and REST APIs
• Create a responsive frontend with vanilla JavaScript for browsing commit history, diffs, and AI interaction metadata

Usage

mem ui                    # Start UI and open browser
mem ui --port 9000        # Use custom port
mem ui --no-browser       # Don't auto-open browser
mem ui --loc /path/to/project  # Specify project directory