feat: Stop with checkpoint save

[entmike]

Summary

Adds a 'Save checkpoint before stopping' option in the job stop confirmation dialog. When enabled, the training loop gracefully saves a checkpoint before terminating, instead of sending SIGINT.

Changes

UI - Stop with Checkpoint Save

• ConfirmModal: Added support — renders an optional checkbox in confirmation dialogs
• JobActionBar: Stop dialog now shows a 'Save checkpoint before stopping' checkbox. When checked, calls ; otherwise calls regular
• jobs.ts: Added function and imported in JobActionBar
• New API route: — sets both and flags in the DB. The training loop's picks them up at the end of the current step:
  1. sees , saves the checkpoint, clears
  2. sees , marks job as stopped, raises Exception to break the training loop

Training Loop - Graceful Stop Logic

• DiffusionTrainer.py & UITrainer.py: Stop watcher threads now check flag before sending SIGINT. If is set, the loop completes the current step, saves the checkpoint, then stops cleanly — no forced termination.

Testing

• ✅ Tested locally — stop with checkpoint save works as expected