Make nodes collapsible with double-click functionality

[mentatbot]

This PR implements collapsible nodes in the repository visualization:

• By default, only directories and files are visible
• Double-clicking a file expands it to show all internal components
• Double-clicking a directory shows/hides contained files
• Double-clicking again collapses the expanded node
• Visual indicators (+ or -) show expandable nodes and their current state
• Added distinct colors for different component types

Closes #29

---

🤖 See my steps and cost here ✨

• Wake on any new activity.

[mentatbot]

I've implemented the collapsible node feature as requested. Here's what the changes provide:

• By default, only directories and top-level files are visible
• Double-clicking a file expands it to show all its internal components (classes, functions, etc.)
• Double-clicking a directory shows or hides its contained files
• Double-clicking an expanded node collapses it
• Visual indicators (+ or -) show which nodes are expandable and their current state
• Added distinct colors for different component types for better visual differentiation
• Added a small hint at the bottom of the visualization about the double-click functionality

The implementation maintains all existing functionality (selection, drag-and-drop, zooming) while adding this new feature.

Let me know if you'd like any adjustments to the implementation!