Refactor localizemedia: add multi-threading, ETA, and resume support

[julius-gun]

Description

This PR refactors the localizemedia add-on from a single-file synchronous script into a modular, multi-threaded package. These changes significantly improve performance for users with large collections and provide a more resilient UX.

Key Improvements

• Multi-threading: Implemented ThreadPoolExecutor (5 workers) to download media in parallel.
• Progress & ETA: Added a real-time ETA calculation and progress updates to the progress bar.
• Resume Capability: If a process is interrupted or cancelled, users can now "Resume Remaining" notes without restarting the entire batch.
• Reliability:
  • Added domain-specific rate limiting/locking to avoid 429 errors.
  • Improved URL normalization (handling special characters like &).
  • Added a session cache to prevent redundant downloads of the same URL within a single batch.
• Safety: Implemented explicit locking for Anki collection access to ensure thread safety.

Architectural Changes

Moved logic into a src/ sub-package:

• media_handler.py: Network requests and file writing.
• note_logic.py: Regex parsing and note modification.
• ui_commands.py: Background operations and UI interaction.
• stats.py: Thread-safe counters and error logging.

Testing

• Verified with notes containing multiple images and audio tags.
• Tested "Cancel" and "Resume" functionality.
• Verified that undo entries are correctly merged upon completion.