feat: support Flash Attention 2 across install paths

[niangao2331]

Summary

• enable Flash Attention 2 self-check based selection for source, portable, and Docker installs
• add portable flash-attn/triton install + recovery flow, including install_flash_attn.bat
• update Docker/source install scripts and docs for verified FA2 with xformers/SDPA fallback

Closes #26

Tests

• python -m py_compile mikazuki\portable_utils.py setup_environment.py mikazuki\app\api.py
• python -m pytest tests\test_portable_utils.py tests\test_train_routing.py
• PowerShell parser check for install/build launcher scripts
• bash -n install.bash

Note: full python -m pytest tests currently has unrelated existing failures in pinned sd-scripts commit expectations and train-log auto-open mocking.