Auto-open single text file in directory

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