feat: Enhance currency handling and validation across scenarios

- Updated form template to prefill currency input with default value and added help text for clarity.
- Modified integration tests to assert more descriptive error messages for invalid currency codes.
- Introduced new tests for currency normalization and validation in various scenarios, including imports and exports.
- Added comprehensive tests for pricing calculations, ensuring defaults are respected and overrides function correctly.
- Implemented unit tests for pricing settings repository, ensuring CRUD operations and default settings are handled properly.
- Enhanced scenario pricing evaluation tests to validate currency handling and metadata defaults.
- Added simulation tests to ensure Monte Carlo runs are accurate and handle various distribution scenarios.

templates/scenarios/form.html

@@ -49,7 +49,11 @@
 
       <div class="form-group">
         <label for="currency">Currency</label>
-        <input id="currency" name="currency" type="text" maxlength="3" value="{{ scenario.currency if scenario else '' }}" />
+        {% set currency_prefill = scenario.currency if scenario and scenario.currency else default_currency %}
+        <input id="currency" name="currency" type="text" maxlength="3" value="{{ currency_prefill or '' }}" placeholder="{{ default_currency or '' }}" />
+        {% if default_currency %}
+          <p class="field-help">Defaults to {{ default_currency }} when left blank.</p>
+        {% endif %}
       </div>
 
       <div class="form-group">