feat: add Playwright-backed ui browser sessions

[TomProkop]

Summary

• add a Playwright-backed browser session platform and new txc ui command group
• extract shared URL-building logic into Core and wire the new feature into the CLI host and solution
• add focused Playwright/UI tests plus CP1 setup and migration documentation

Validation

• dotnet build src/TALXIS.CLI/TALXIS.CLI.csproj
• dotnet test tests/TALXIS.CLI.Platform.Playwright.Tests/TALXIS.CLI.Platform.Playwright.Tests.csproj --no-build
• dotnet test tests/TALXIS.CLI.Features.Ui.Tests/TALXIS.CLI.Features.Ui.Tests.csproj --no-build
• dotnet test tests/TALXIS.CLI.Tests/TALXIS.CLI.Tests.csproj --no-build --filter "FullyQualifiedName~Architecture"
• dotnet run --project src/TALXIS.CLI/TALXIS.CLI.csproj --no-build -- ui --help