Finalize WinUI migration polish

[Xueyang-Song]

Summary

This PR lands the full WinUI migration polish pass and supporting workflow updates.

• Adds the WinUI semantic design system, app-owned primitives/components, bundled fonts, and refreshed shell/page styling.
• Reworks activity feed rendering around typed GitHub event payloads, inline in-app navigation targets, and design-lab mock coverage.
• Improves PR/issue/comment/reaction surfaces, removes Pro feature gating, and continues moving legacy Octokit compatibility behind app-owned models.
• Updates app and website branding assets to the latest JitHub logo set.
• Adds WinUI screenshot automation plus Windows App CLI / Microsoft Store CLI workflow documentation and helper scripts.

User Impact

• The app should feel more cohesive and modern across shell, dashboard, repo, issue, PR, settings, and login surfaces.
• Activity cards now expose contextual in-app links instead of generic browser launch behavior.
• Existing app/package identity is preserved while icons and website assets are refreshed.
• Developer workflow now includes repeatable WinUI debug launch and screenshot proof tooling.

Validation

• dotnet build .\JitHub.slnx -c Debug -p:Platform=x64 -v:minimal
• dotnet build .\JitHub.Web\JitHub.Web.csproj -c Debug -v:minimal
• .\capture-winui-design.ps1 -Configuration Debug -TargetPlatform x64 -Themes 'dark' -Targets 'shell'

Notes

• Generated screenshots remain local artifacts and are not committed.
• .codex/ and *.backup.png are ignored to avoid local environment and backup-file leakage.