Add log file download button on Web Dashboard

[mx57]

Added a log download button on the Live Log Monitor section of the Web Dashboard.

Key changes:

• Implemented DownloadLog(string type) action in HomeController.cs to stream the active general or trades log file safely using FileShare.ReadWrite.
• Updated Dashboard.cshtml and dashboard.js with a "Скачать лог" button that dynamically binds to the selected log type (general vs trades).
• Added comprehensive unit tests in IntelliTrader.Trading.Tests/HomeControllerTests.cs covering 404 handling and successful file stream responses.
• Updated magda_agent_system/agent_tasks.json marking web-dashboard-log-download as done and added a replenishment todo task to preserve schema requirements.

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PR created automatically by Jules for task 3453403094471308687 started by @mx57

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