feat: Add Markdown File Support for Audiobook Generation

[brianxiadong]

Summary

• Added comprehensive support for Markdown (.md/.markdown) files in the audiobook generation workflow
• Implemented automatic chapter parsing based on markdown headers with hierarchical structure support
• Added YAML frontmatter metadata extraction for title, author, description, and publication date
• Updated UI components to include markdown files in file dialogs and drag-drop functionality

Features Added

• File Type Detection: Automatic recognition of .md and .markdown file extensions
• Header-based Chapter Parsing: Converts markdown headers (# ## ### etc.) into navigable chapters
• Hierarchical Structure: Maintains proper parent-child relationships based on header levels
• Metadata Extraction: Parses YAML frontmatter for book metadata (title, author, description, date)
• Smart Content Selection: Auto-selects chapters with significant content (>500 characters)
• UI Integration: Updated file dialogs, drag-drop areas, and error messages to include markdown support

Technical Implementation

• Extended HandlerDialog class to handle markdown file type
• Added _preprocess_markdown_content() method for parsing markdown structure
• Implemented _build_markdown_tree() for creating hierarchical chapter tree view
• Added _get_markdown_selected_text() for text extraction with chapter markers
• Updated file type detection logic across GUI components

examples:

[denizsafak]

This is looking very good, thank you so much! I will definitely merge this into the project after some testing 😋