Auto-open single text file in directory

[audreyfeldroy]

When pad is launched in a directory containing exactly one text file, open that file automatically instead of showing an empty editor.

Implementation notes

• On startup, check if the target directory contains exactly one text file
• Use existing logic for identifying text files (likely based on extension or mime type)
• If condition is met, open that file in the editor
• If directory has 0 or 2+ text files, use current behavior