Check for Windows updates on startup

[puppyben1]

What changed

• Keep the Windows welcome-page Check for Updates action wired to the shared update flow.
• Trigger one automatic update check after the WinUI root has loaded.
• Reuse the existing release dialog so only an available update opens a modal; up-to-date and failure results remain status-only.
• Guard the startup trigger so repeated Loaded events do not start duplicate checks.

Why

Windows users should be able to check from the home page and should learn about new releases on the first app launch without navigating into Settings.

Validation

• scripts/verify-windows-boundaries.ps1
• scripts/build-windows.ps1 -Configuration Release -BuildWinUI
• ctest --test-dir windows/build-windows -C Release --output-on-failure (18/18 passed)

[1lck]

基线错了哦 这个pr还在草稿状态吗 @puppyben1