Offer a diagnostics download in Settings

[theg1239]

Summary

• The Diagnostics panel only copies logs to the clipboard, which fails in some restricted environments.
• Provide a "Download log" action that saves a .txt file locally.

Why it matters

• Improves supportability and gives users a reliable way to attach logs to bug reports.

Acceptance Criteria

• Add a secondary button in apps/desktop/src/pages/Settings.tsx alongside "Copy log" to trigger a file download.
• Use the Tauri dialog.save API or @tauri-apps/api/fs to write the compiled diagnostics string to disk.
• Display success/error feedback similar to the copy action.

Additional context

• The diagnostics payload already comes from exportDiagnostics; reuse that helper.

[Ai-Chetan]

@theg1239 I can work on this,
Please assign it to me!