feat(review): accept "Stk." in a unit correction and store the ERP unit

[Fluory]

Goal

The review screen shows the unit pcs as „Stk.“ (#77). A clerk who types „Stk.“ into a unit correction today stores the non-canonical text, and the ERP contract exports it as written. Found in the fresh review of PR #95 (note 5); #95 adds a hint below the correction field as a stopgap.

Acceptance criteria

• A unit correction accepts the German labels („Stk.“, „Stück“) and stores the canonical ERP unit (pcs); other units stay as entered
• The audit event keeps what the clerk typed and what was stored
• The hint below the unit correction field is removed or reworded

Test plan

Criterion	Check
normalization	unit test of the correction input mapping
stored + exported value	integration test: correction „Stk.“ → export payload pcs

Security/Privacy affected?

No.

[Fluory]

Claimed by @Fluory on branch claude/feat-unit-correction-96 – draft PR follows within the hour.