fix(tui): resolve cross-platform root detection and fallback paths in pickers

[Piyush0049]

Summary

This PR addresses two cross-platform path handling issues in the TUI file and directory picker components to ensure reliable behavior on Windows and POSIX systems.

1. Cross-Platform Root Directory Boundary Detection

• Previous behavior: The TUI checked d.currentDir != "/" to decide whether to append the .. (parent directory) option.
• Issue: On Windows, root directories start with drive letters (such as C:\). Since C:\ does not equal /, the TUI treated C:\ as a non-root folder and appended a .. entry pointing back to C:\, causing an infinite navigation loop.
• Fix: Replaced hardcoded POSIX root checks with filepath.Dir(d.currentDir) != d.currentDir. This leverages the standard library to verify if a directory is its own parent, which works reliably on Windows, Linux, and macOS.

2. Cross-Platform Directory Fallback (/ to .)

• Previous behavior: When os.Getwd() returned an error or empty string, the TUI fell back to workingDir = "/".
• Issue: Hardcoding / as a fallback causes invalid path resolutions on Windows systems when execution context is lost.
• Fix: Updated the fallback path from / to .. This aligns with the existing fallback pattern used in file_picker.go and ensures safe resolution relative to the process execution directory across all platforms.

Verification

• Verified with golangci-lint run (0 issues).
• Verified unit test suite with go test ./pkg/tui/dialog.