Refactor: Eliminate code duplication for currency conversion and item pricing

- Add static calculateItemValueSC() and getItemValueSC() helpers to MournbladeCYD2Utility
  to centralize currency conversion logic (1 PO = 100 SC, 1 PA = 10 SC)
- Refactor computeRichesse() and computeValeurEquipement() in Actor to use shared helpers
- Add localizeAllegiance Handlebars helper to display allegiance values localized
  (tous->Tous, chaos->Chaos, loi->Loi, betes->Bêtes, elementaires->Élémentaires)
- Add joinPredilections helper to fix comma display after single Predilection
- Create BaseItemWithPriceDataModel base class for items with pricing fields
  (prixpo, prixca, prixsc, rarete, quantite, equipped)
- Update arme, equipement, protection, monnaie models to extend base class
- Update actor-sheet and creature-sheet templates to use new helpers
- Update partial-item-prix.hbs to display total item value in SC
- Add item-base-sheet.hbs template for future item sheet inheritance

Fixes:
- Allegiance values now display properly localized in Dons & Pactes tabs
- Predilections no longer show trailing comma with single entry
- Equipment value totals now update correctly when items are added/modified
- Currency conversion logic centralized and consistent across the system

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

templates/creature-sheet.hbs

@@ -266,9 +266,7 @@
                   <a data-action="rollCompetence" data-attr-key="tochoose">{{skill.name}}</a>
                 </span>
                 <span class="predilection-text">
-                  {{#each skill.system.predilections as |pred key|}}
-                    {{#if (and pred.acquise (not pred.used))}}{{pred.name}}{{#unless (eq key (subtract skill.system.predilections.length 1))}}, {{/unless}}{{/if}}
-                  {{/each}}
+                  {{joinPredilections skill.system.predilections}}
                 </span>
               </div>
               <select class="status-small-label color-class-common edit-item-data competence-niveau"