Add log file download button on Web Dashboard

[mx57]

Implemented the missing log file download functionality on the Web Dashboard (web-dashboard-log-download task):

1. HomeController.cs: Added DownloadLog(string type = "general") GET endpoint that safely streams the latest general (*-general.txt) or trade (*-trades.txt) log file as a .txt attachment using FileShare.ReadWrite access.
2. Dashboard UI (Dashboard.cshtml & dashboard.js): Added "Скачать лог" (Download log) buttons to both the Live Log Monitor and Live Trade Logs Terminal card headers, and synchronized the download link href dynamically when switching log types.
3. Unit Tests (HomeControllerTests.cs): Created new unit tests verifying NotFound responses for missing files and FileStreamResult streaming for active log files.
4. Task Pool & Validation: Marked task web-dashboard-log-download as "done" in agent_tasks.json and validated schema conformance with validate_agent_tasks.py.

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PR created automatically by Jules for task 8716738200504241616 started by @mx57

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