feat: startup splash with init progress

[keyldev]

• run DB migrations + event ingestion on a splash window off the UI thread instead of blocking synchronously before Avalonia starts, which looked like a window-less freeze on a slow first run
• StartupViewModel shows status/progress and offers Retry/Exit on failure, then swaps in the main window once init completes
• Android bootstrap (MainApplication) left unchanged

Summary

• run DB migrations + event ingestion on a splash window off the UI thread instead of blocking synchronously before Avalonia starts, which looked like a window-less freeze on a slow first run
• StartupViewModel shows status/progress and offers Retry/Exit on failure, then swaps in the main window once init completes
• Android bootstrap (MainApplication) left unchanged

Related

Type

• Bug fix
• Feature
• Refactor / cleanup
• Docs / CI
• Other:

Checklist

• Builds with 0 warnings (TreatWarningsAsErrors=true).
• Tests pass (dotnet test); new Core logic has unit tests.
• No layering violation — App references Core only (Infrastructure / Video / Devices wired via DI in a head).
• Scope stays within one phase (didn't pull work from a later phase's "Не входит").
• README / docs updated if public commands, options, or setup changed.

Platforms tested

• Windows
• Linux
• macOS
• Android
• iOS
• CI build only

Screenshots / notes