Add optional desktop GUI entry point

[AthillaZaidan]

Summary

Adds a desktop GUI entry point for MarkItDown so users can convert one local file into a Markdown output file without using
the CLI directly.

Changes

• Add markitdown-gui console script
• Add PySide6-based dark desktop GUI
• Add native file picker for input and output paths
• Reuse existing MarkItDown Python API for conversion
• Add PDF dependencies to default install so GUI PDF conversion works out of the box
• Add GUI-focused tests
• Document GUI setup and run commands in README files

How to run

cd packages/markitdown
uv pip install -e .
markitdown-gui

If using an already active virtual environment:

cd packages/markitdown
pip install -e .
markitdown-gui

## Verification

- uv run --with pytest pytest tests/test_gui.py -q
- uv run --with pytest pytest tests/test_cli_misc.py -q
- Manual PDF conversion smoke via GUI helper

[AthillaZaidan]

@microsoft-github-policy-service agree