Add DXF to ACODE conversion tab with arc segmentation

[peterwolf-pl]

Summary

• add DXF → ACODE tab in the web UI with PID note, arc-seg controls, preview, and sender integration
• expose backend DXF conversion endpoint that runs acode.py and acodeviz for previews
• support optional arc segmentation in acode.py to break curves into smaller commands

Testing

• not run (not requested)

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Codex Task