Serve operator UI at root route

[cbusillo]

Summary

• serve the operator UI app shell from /
• keep /ui and /ui/* as compatibility routes for the existing UI path and client routes
• preserve /v1/... as the stable API namespace
• document the root UI route and add focused service coverage

Verification

• uv run python -m unittest tests.test_service.LaunchplaneServiceTests.test_root_route_serves_ui_shell_without_authentication tests.test_service.LaunchplaneServiceTests.test_ui_route_serves_static_shell_without_authentication tests.test_service.LaunchplaneServiceTests.test_ui_route_falls_back_to_shell_for_nested_paths tests.test_service.LaunchplaneServiceTests.test_ui_asset_route_rejects_parent_directory_segments passed
• uv run python -m unittest passed, 422 tests
• npx pnpm@10.10.0 --dir frontend validate passed
  • local Node emits the expected engine warning because this machine is on Node v25; CI uses Node 22
• git diff --check passed