fix(app): allow browsing absolute paths on Windows

[usvimal]

Summary

Fixes #6490 - Windows users can now browse and open projects outside their home directory.

Problem

On Windows, the directory selector dialog was limited to browsing within C:\Users\<username>. This made it impossible to:

• Open projects on other drives (D:, E:, etc.)
• Access folders outside the user profile

Solution

• Added isAbsolutePath() function to detect Windows drive letters (`C:\ in the directory selector -> shows folders on D: drive
• Type C:\Projects -> searches within C:\Projects
• Existing relative path behavior unchanged