Split Windows launchers and add China mirror fallback

[EveningStudy]

What changed

• split the Windows entry point into a web UI launcher and a repeatable dependency installer/repair tool
• added a root-level mirrors.json with domestic-first PyPI, Hugging Face, PyTorch, GitHub, uv, and Python download sources
• added automatic fallback and resume behavior across Windows, Linux, and in-app backend/model installation
• updated only the related installation and troubleshooting documentation

Why

The UI launcher should fail fast when the Core environment is missing or damaged, while dependency installation must be safe to rerun. Most users are in mainland China, so model and dependency downloads need configurable domestic mirrors with automatic fallback.

Validation

• 120 pytest tests passed
• Ruff lint and format checks passed
• Windows PowerShell and Linux shell syntax checks passed
• both Windows console executables built and passed their self-tests
• source distribution contains both executables and mirrors.json