Adds local persistence for download tasks using libtorrent's native resume data APIs and SQLite for metadata. Tasks automatically resume on restart. Data stored in WinUI3's recommended ApplicationData::Current().LocalFolder().

Changes

New: TorrentStateManager (Core/torrentCore/)

• SQLite-backed storage for task metadata (magnet URI, save path, progress, status)
• Session state persistence via session_state() / apply_settings()
• Resume data via write_resume_data() / read_resume_data()
• Bencode-based import/export for backup

LibtorrentHandle updates

• Track task ID ↔ torrent handle mappings
• Handle save_resume_data_alert, metadata_received_alert
• Save resume data on torrent events (finish, pause, error)
• AddTorrentFromResumeData() for restoring saved torrents

P2PManager updates

• Initialize state manager on startup
• LoadAndResumeSavedTasksAsync() resumes incomplete downloads
• Expose GetAllTasks(), ExportTasksAsync(), ImportTasksAsync()

TasksViewModel updates

• Load saved tasks on init with progress/status display
• Add ExportCommand / ImportCommand

Storage Schema

CREATE TABLE tasks (
    task_id TEXT PRIMARY KEY,
    magnet_uri TEXT NOT NULL,
    save_path TEXT NOT NULL,
    name TEXT,
    added_timestamp INTEGER,
    total_size INTEGER,
    downloaded_size INTEGER,
    status INTEGER,  -- 0=pending, 1=downloading, 2=paused, 3=completed, 4=failed
    resume_data BLOB
);

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