Add a currency-country assignment widget to CurrencyDetailDialog

Add a currency-to-country assignment widget to CurrencyDetailDialog, mirroring the existing CalendarAssignmentWidget pattern used for currency_calendar (see projects/ores.qt/refdata/src/CurrencyDetailDialog.cpp:690 and projects/ores.qt/api/include/ores.qt/CalendarAssignmentWidget.hpp): a reusable widget embedded in the currency detail dialog that lists a currency's assigned countries and lets a user assign/revoke them, backed by the existing ores_refdata_currency_countries_tbl junction (projects/ores.sql/create/refdata/refdata_currency_country_create.sql) and its NATS messaging surface.

currency_country (a currency can be issued by more than one country, e.g. EUR spans DE/FR/IT/ES/AT/FI) is a sibling junction to currency_calendar — both are many-to-many currency assignments — but only currency_calendar has a Qt UI. currency_country has none anywhere in the application: no way to view or manage which countries issue a currency. The gap surfaced while writing the QA test scenario for DQ-publish the calendar reference data (which wired currency_country into the DQ bundle-publish pipeline) — that scenario had to fall back to the generic Publication Audit dialog to confirm data landed, instead of a dedicated screen, because none exists.

• projects/ores.qt/refdata/src/CurrencyDetailDialog.cpp:690 — the CalendarAssignmentWidget embedding this widget should mirror.
• projects/ores.qt/api/include/ores.qt/CalendarAssignmentWidget.hpp
• projects/ores.sql/create/refdata/refdata_currency_country_create.sql

• ores.codegen.junction: ores.refdata.currency_country
• Story: Finish integrating calendar reference data: XML export and DQ publish
• Task: DQ-publish the calendar reference data