fix(accounts): default CreateChartAccount.currencies to None

The UI omits currencies so the Open directive is written unconstrained,
but the model defaulted currencies to ["EUR","SATS","USD"], so Pydantic
refilled them and the endpoint passed the constraint through — every
admin-created account got a currency-constrained Open (which would
reject postings in other currencies, the same CAD/GBP/JPY bean-check
class we hit on user accounts). Default to None so omission reaches
add_account and the directive is unconstrained; an explicit list still
works for API callers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

models.py

@@ -51,7 +51,11 @@ class CreateAccount(BaseModel):
 class CreateChartAccount(BaseModel):
     """Admin-created chart-of-accounts entry written to accounts/chart.beancount."""
     name: str  # Full hierarchical account name, e.g. "Expenses:Services:Domain"
-    currencies: list[str] = ["EUR", "SATS", "USD"]
+    # Optional currency constraint. Omitted by the UI: an Open directive needs
+    # no currency list, and constraining it would reject postings in other
+    # currencies (the CAD/GBP/JPY bean-check errors we saw on user accounts).
+    # None → unconstrained Open; a list → explicit constraint for API callers.
+    currencies: Optional[list[str]] = None
     description: Optional[str] = None