BUG-T15: Web UI port/config investigation

[SoundBlaster]

Summary

• investigate the reported Web UI startup failure when both --web-ui-port and --web-ui-config are passed
• add explicit runtime diagnostics for CLI-over-config precedence and collision guidance
• update Web UI setup docs to avoid misleading combined-flag MCP example
• add regression tests for precedence and collision hint behavior

Validation

• pytest
• ruff check src/
• mypy src/
• pytest --cov (91.39%)

Artifacts

• validation report: SPECS/ARCHIVE/BUG-T15_WebUI_Port_Config_Investigation/BUG-T15_Validation_Report.md
• review report: SPECS/ARCHIVE/_Historical/REVIEW_bug_t15_webui_port_config.md