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

[noezhiya-dot]

This PR adds a desktop GUI entry point, which is a reasonable feature. However, the README says "This fork includes a desktop GUI" — if this is meant for upstream, the docs should not reference "this fork". Also, adding a GUI dependency (tkinter or similar) to a CLI tool increases the install surface. Consider making the GUI an optional extra (e.g., pip install markitdown[gui]) rather than a default entry point.