Migrate crash reporting to Devolutions Cloud and port to Avalonia

[Gabriel Dufresne (GabrielDuf)]

• Replaces the old crash-report flow (opening a browser URL) with a POST to https://cloud.devolutions.net/api/senderrormessage, so reports land in a structured way without requiring the user to leave the app.
• Adds CrashReportWindow (WinUI + Avalonia) — a modal dialog shown on the next normal launch after a crash, letting the user optionally attach their email and additional context before sending or dismissing.
• Ports the full crash-handler pipeline to the Avalonia shell: CrashHandler (cross-platform, no user32.dll), AppDomain.CurrentDomain.UnhandledException hook in Program.Main, and the modal startup gate in App.axaml.cs.

How it works

1. When the app crashes, CrashHandler.ReportFatalException writes the crash report to %TEMP% UniGetUI_pending_crash.txt and exits.
2. On the next launch, StartupAsync detects the file, deletes it, and shows CrashReportWindow as a modal dialog over the main window — the rest of startup is blocked until the user sends or dismisses the report.
3. Sending POSTs a JSON payload { email, message, errorMessage, productInfo } to the Devolutions endpoint; network failures are swallowed silently.