feat(train): Streamlit UI for layout import, train, and eval (#101)

[loootte]

Summary

Implements #101 training UI on top of the #95 Framework.

UI (Streamlit)

• Import real .enpu.json -> layout GT (layout_gt export + validate)
• Sample list with validation status / systems / splits
• One-click train (subprocess, progress poll, cancel, log tail, loss chart)
• One-click eval with L2 IoU + L3 line-level metrics
• History scan of runs/

Code

• train/ui/app.py
• train/enpu_train/ui_backend.py
• scripts/run_ui.py
• tests for backend + subprocess train

Run

cd train
pip install -r requirements.txt
$env:PYTHONPATH = \"$PWD;$PWD\..\core\"
python scripts\run_ui.py

Closes #101