Feature: Custom output directory for saved Markdown files

[BOSSincrypto]

Feature Request

Currently, when saving converted Markdown files, the app prompts a save dialog for each file individually. For batch workflows, this is slow.

Expected behavior

• Allow user to set a default output directory in settings
• When "Save All" is triggered, all files are automatically saved to that directory with their original filename + .md extension
• Optionally: confirm before overwriting existing files

Why this matters

Power users converting dozens of documents per session need a non-interactive batch save mode.