Add sender start offsets and simulator enhancements

[peterwolf-pl]

Summary

• allow the sender to start from an arbitrary line, display last OK line, and expose time estimates
• add manual serial command sending plus simulator overlay on the Printer tab with line highlighting
• compute motion estimates and trajectory data for loaded ACODE, reusing in the new current-ACODE viz endpoint

Testing

• python -m compileall ui_sender.py

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Codex Task