Implement Download Manager

[mirkobrombin]

• DownloadManager manages queue, workers, config, and global state
• DownloadTask with URL, destination path, state, progress, and retries
• Thread-safe queue (e.g. crossbeam,tokio::sync)
• Configurable workers, auto or manual start
• Parallel downloads respecting max workers via semaphore or bounded channel
• Add one or more URLs with optional custom path for dstination
• Track progress: bytes downloaded, total, speed, ETA (clients must be able to subscribe to live updates)
• Retry on failure with basic backoff
• Cancel single task or all tasks
• Temp file handling and atomic move on completion
• Export/import queue state for crash recovery
• Support event/callback hooks on task state changes
• Allow injecting custom download backends (HTTP client abstraction)
• (bonus) Might be useful to expose clean API for external crates (this might be a dedicated library, actually)