feat(gui): add live session model selection

[zhnt]

Summary

• expose the redacted AppHost model catalog to an attached GUI session
• add session-local model selection with authority and running-state checks
• keep semantic model-control rejection separate from transport reconnect behavior
• preflight the same attachment-scoped model contract in the live desktop acceptance launcher

Contract changes

• add session/models and session/model/select to loushang.app/v1
• keep provider credentials and direct models.json access outside the WebView boundary

Validation

• GUI: 39 tests passed
• Rust: 22 tests passed
• Python focused suites: 86 tests passed
• Ruff and mypy passed
• live provider acceptance, automatic reconnect, and controller cleanup verified manually