Implement parallel download

[LainX84]

Description of Changes

This Pull Request introduces concurrent image downloading, resolves a critical startup IndexError crash, and significantly increases downloader robustness by improving the HTTP retry and rate-limiting logic.

Key Changes

1. Bug Fix & URL Normalization

• IndexError Resolution: Fixed IndexError: list index out of range in Crawler._generate_album_pages that occurred when downloading single-page albums. The crawler now parses pagination parameters safely, finds the maximum page using max(), and dynamically builds the page list rather than relying on a fragile hardcoded index ([-2]).
• URL Normalization: Added URL normalization inside Crawler and AlbumDownloader to guarantee a trailing slash / and strip existing query parameters (such as ?p=...). This prevents regex pattern matching mismatches against gallery HTML links.

2. Parallelized Image Downloads

• Multithreading: Integrated ThreadPoolExecutor in AlbumDownloader to download the pictures of a gallery page concurrently.
• Concurrency Configuration: Added a MAX_WORKERS setting (defaulting to 5) in config.py to customize download speed while keeping requests polite.
• Thread Safety:
  • Added synchronization locks (threading.Lock) inside ProgressManager and LiveManager to prevent terminal UI glitches or exceptions during concurrent progress updates.
  • Decoupled requests.Session header mutation by passing request-specific headers directly to each request, ensuring thread-safe operations.

3. Improved HTTP Retry & Rate-Limiting Logic

• Refactored fetch_with_retries to specifically handle rate-limit status codes 429 (Too Many Requests) and 509 (Bandwidth Exceeded). The downloader now logs a rate-limit warning, sleeps for RATE_LIMIT_SLEEPING_TIME (60 seconds), and retries the download.
• Introduced immediate exit for non-recoverable HTTP errors (like 404 Not Found) to prevent redundant retry attempts.

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Files Changed

• src/config.py: Added the MAX_WORKERS configuration.
• src/crawler/crawler.py: Fixed the IndexError, normalized URLs, and added the thread-safe get_reloaded_page() helper.
• src/downloader/album_downloader.py: Redesigned downloading logic to run concurrently.
• src/downloader/download_utils.py: Added thread-safe headers support and improved retry logic (handling of 429/509/404).
• src/managers/progress_manager.py & src/managers/live_manager.py: Integrated thread-safety locks for UI rendering.