fix: use case-insensitive path comparison on Windows to prevent duplicate projects

[usvimal]

Summary

On Windows, file paths are case-insensitive but the project deduplication check used strict equality (===). This caused the same directory opened with different casing (e.g., C:\Users vs c:\Users) to appear as separate entries in the sidebar.

Changes

• Added isWindows() helper to detect Windows platform
• Added pathsEqual() helper for case-insensitive path comparison on Windows
• Updated all path comparisons in projects.open/close/expand/collapse/move to use pathsEqual()

Testing

Tested on Windows 11 - opening the same directory with different path casing no longer creates duplicate sidebar entries.