feat: Add log file download capability to Web Dashboard

[mx57]

This change completes the web-dashboard-log-download task:

1. Adds [HttpGet] DownloadLog(string type) endpoint to HomeController in IntelliTrader.Web.
2. Adds Download Log export buttons to both terminal card headers in Dashboard.cshtml.
3. Dynamically updates download URLs when switching log types in dashboard.js and Dashboard.cshtml.
4. Adds unit tests in HomeControllerDownloadLogTests.cs verifying file streaming and error handling.
5. Updates magda_agent_system/agent_tasks.json status to "done".

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PR created automatically by Jules for task 40765375141917693 started by @mx57

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