Implement the web UI server using the current Zig I/O networking APIs

Problem

The UI server API and CLI flags exist, but server startup, client acceptance, and event broadcasting are currently stubs. Running with --ui therefore advertises a URL without serving a functional interface.

Proposed work

• Implement HTTP serving with the current Zig I/O networking APIs.
• Add a streaming event transport for run, suite, and test lifecycle events.
• Serve or bundle a minimal test-results interface.
• Handle startup failures, shutdown, and client disconnects safely.
• Add integration tests for server startup and event delivery.

Acceptance criteria

• zig-test --ui serves a working page at the reported address.
• Test events appear in the UI while a run is active.
• Ctrl+C and normal completion shut the server down cleanly.
• UI tests run reliably in CI without fixed-port conflicts.

Completed by #26, merged via rebase after the full Linux, macOS, and Windows CI matrix passed.