Move all dependency-free business logic (metadata schema, rounding/parsing
helpers, item extraction, and the XRechnung/ZUGFeRD XML generators plus the
PDF/A output-intent helpers) out of main.py into a new invoice_logic.py that
imports only pydantic and pikepdf. This makes the logic unit-testable without
a Streamlit runtime. Pure cut-and-paste, no behavior change; main.py now
imports these names from invoice_logic instead of defining them locally.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
dfuhrmann3