fix: eliminate black-square flash on startup, start cleanly in tray

[Daolyap]

What

Fixes the black square that briefly appears on app startup and ensures the app starts minimized to the system tray with no visible window.

Why

App.xaml used StartupUri="MainWindow.xaml", which makes WPF Show() the window during startup. The StartInTray check (and Hide()) only ran afterward in the Loaded handler — which fires after the window is shown and composed. With WindowStyle="None" + WindowChrome, the brief frame before the WPF content painted appeared as a black square that then vanished. It was an unavoidable Show-then-Hide flash.

How

• App.xaml — removed StartupUri (which forces Show()); set ShutdownMode="OnExplicitShutdown", the correct mode for a tray app whose main window may never be shown (otherwise WPF could quit when a transient dialog like the editor closes).
• App.xaml.cs — OnStartup now creates MainWindow explicitly and calls InitializeApplication() instead of relying on StartupUri.
• MainWindow.xaml.cs — replaced the Loaded handler with InitializeApplication(). When StartInTray is set it calls WindowInteropHelper.EnsureHandle() to realize the native HWND (needed for global hotkey registration) without painting the window; otherwise it shows the window normally. Hotkeys bind after the handle exists in both paths.

Testing

• dotnet build clean (0 warnings / 0 errors).
• dotnet test — 109/109 service tests pass.
• Launched the built exe: process stays alive with MainWindowHandle: 0, and an EnumWindows sweep confirmed zero visible top-level windows for the process — starts silently in the tray, no black-square flash.

Reviewer notes

• No automated UI coverage exists (per DEVELOPER.md). Manual check worth doing on Windows: launch, then tray icon → "Show Window", and confirm a capture hotkey (Print Screen) still fires.
• WindowStartupLocation="CenterScreen" is computed at handle-creation time inside CreateSourceWindow, so it is still honored even when the handle is realized early via EnsureHandle().

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