chore(deps): modernize September 2026 dependency stack

[Proxicon]

Closes #27

Summary

• Updates direct .NET dependencies, SDK pin, Docker SDK base images, and GitHub Actions to current stable releases.
• Migrates System.CommandLine to stable 2.x, MudBlazor to 9.10, xUnit to 3.0/Microsoft Testing Platform, and removes unused deprecated SharpCompress references.
• Replaces deprecated SIPSorceryMedia.Encoders with SIPSorceryMedia.FFmpeg 10.0.16.
• Removes pre-release OpenTelemetry Process instrumentation rather than retaining a pre-release dependency. Runtime instrumentation remains enabled.

Validation

• dotnet restore, clean Release build (0 warnings, 0 errors), and dotnet test --filter category!=compose: 1,860 passed; 4 expected live-environment skips.
• Source Compose OIDC smoke: 2 browser tests, client enrollment/authentication, CLI and MCP archive checks passed.
• Client Docker image rebuild and MCP HTTP image smoke passed.
• Public disclosure, layout, product-version, and release-validation checks passed.
• NuGet deprecated/vulnerable package audits are clean.

Review note

SIPSorceryMedia.FFmpeg requires FFmpeg 8.1-compatible shared libraries on hosts that run interactive remote support. This is its upstream runtime prerequisite and needs deployment review before enabling that path.

No merge, deployment, release, tag, or package publication has been performed.