feat: Add currency management feature with CRUD operations

- Introduced a new template for currency overview and management (`currencies.html`).
- Updated footer to include attribution to AllYouCanGET.
- Added "Currencies" link to the main navigation header.
- Implemented end-to-end tests for currency creation, update, and activation toggling.
- Created unit tests for currency API endpoints, including creation, updating, and activation toggling.
- Added a fixture to seed default currencies for testing.
- Enhanced database setup tests to ensure proper seeding and migration handling.

templates/partials/base_header.html

@@ -2,6 +2,7 @@
   ("/", "Dashboard"),
   ("/ui/scenarios", "Scenarios"),
   ("/ui/parameters", "Parameters"),
+  ("/ui/currencies", "Currencies"),
   ("/ui/costs", "Costs"),
   ("/ui/consumption", "Consumption"),
   ("/ui/production", "Production"),