Add EML export implementation guide and GetManyRFC2822Task

[bengotow]

Summary

This PR adds comprehensive documentation for EML export functionality and implements the GetManyRFC2822Task in mailsync to enable bulk folder export. It includes a detailed implementation guide for the client-side features that will be built in the Mailspring repo.

Key Changes

• New documentation (EML_EXPORT_IMPLEMENTATION.md): Complete architecture guide covering:

  • Background and community request history
  • Overview of two task types (GetMessageRFC2822Task and GetManyRFC2822Task)
  • Four client-side features (single download, drag-and-drop, forward as attachment, folder export)
  • Detailed mailsync task specifications with JSON schemas
  • Client implementation guide with code examples
  • Testing checklist and validation procedures

• New mailsync task (GetManyRFC2822Task):

  • Exports all messages in a folder to a directory as .eml files
  • Processes messages in chunks of 50 with progress tracking
  • Supports resume on restart by tracking exported count
  • Handles per-message errors gracefully without aborting the export
  • Supports cancellation via should_cancel flag
  • Generates sanitized filenames with format: {index} - {subject} - {date}.eml

• Filename sanitization helper (sanitizeEmlFilename):

  • Replaces filesystem-illegal characters with underscores
  • Truncates subjects to 80 characters
  • Strips trailing dots and spaces (Windows requirement)
  • Zero-pads index to 5 digits for natural sort order
  • Clamps total filename length to 200 chars on Windows for MAX_PATH compliance

• Task routing: Added GetManyRFC2822Task handling in performLocal and performRemote methods

Implementation Details

• Progress is saved to task.data["progress"] after each chunk of 50 messages, enabling real-time progress bar updates in the client
• Final results written to task.data["result"] with summary statistics and error list
• Chunk processing includes 1-second sleep between chunks to prevent blocking other sync operations
• Resume logic reads task.data.progress.exported to determine starting index on restart
• Error handling logs per-message failures but continues processing remaining messages
• Supports both POSIX and Windows path handling with appropriate string conversions

https://claude.ai/code/session_013AQPfia7V9Mxf4NL9peegn