BUG: Symlink attack in filepicker path navigation

[andrinoff]

Describe the bug
Filepicker at tui/filepicker.go:94 uses os.Stat() without checking symlinks. Attacker could create symlinks to read arbitrary directories.

To reproduce

1. Create symlink: ln -s /etc/passwd ~/.config/matcha/link
2. Navigate to symlink path via filepicker
3. Contents of /etc exposed

Expected behavior
Use os.Lstat() to detect symlinks, or explicitly follow only safe links

Matcha version
master

OS
Unix-like

Additional context
File: tui/filepicker.go line 94
Security: symlink directory traversal