[codex] Fix repo manage crash

[Xueyang-Song]

Summary

Fixes the Manage Repository crash reported during Microsoft Store certification.

Root cause

The Manage Repository page materialized selectable repository templates that applied the shared Container style to Grid elements. That style targets Border, so WinUI threw a XAML parse exception when the template loaded after clicking the gear button. In the Store crash report this surfaced as a 0xc000027b fail-fast in Microsoft.UI.Xaml.dll.

Changes

• Use Border for selectable repository templates that apply the Container style.
• Move RepoManagePage to the newer DI-backed RepoManagePageViewModel flow.
• Add guarded load/delete handlers so GitHub/auth/API failures become page status instead of escaping through XAML events.
• Add a regression test that prevents applying the Container style to non-Border elements.

Validation

• dotnet build .\JitHub.WinUI\JitHub.WinUI.csproj -c Release -p:PublishAot=false -p:PublishTrimmed=false
• UI smoke: launched the release app with --page=home, invoked RepoSideBarManage, verified “Manage repositories” appeared and the process stayed alive.
• dotnet test .\JitHub.WinUI.Tests\JitHub.WinUI.Tests.csproj -c Release passed: 99 tests.
• powershell -NoProfile -ExecutionPolicy Bypass -File .\eng\Build-JitHubWinUIStorePackage.ps1 -ProjectPath .\JitHub.WinUI\JitHub.WinUI.csproj -OutputDirectory .\artifacts\store-smoke -BundlePlatforms x64 completed and produced the bundled .msixupload.