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>

modules/models/monnaie.mjs

@@ -1,17 +1,13 @@
 /**
  * Data model pour les monnaies MournbladeCYD2
  */
-export default class MonnaieDataModel extends foundry.abstract.TypeDataModel {
+import { BaseItemWithPriceDataModel } from "./base-item.mjs";
+
+export default class MonnaieDataModel extends BaseItemWithPriceDataModel {
   static defineSchema() {
     const fields = foundry.data.fields;
     return {
-      description: new fields.HTMLField({ initial: "" }),
-      prixpo: new fields.NumberField({ initial: 0, integer: true }),
-      prixca: new fields.NumberField({ initial: 0, integer: true }),
-      prixsc: new fields.NumberField({ initial: 0, integer: true }),
-      rarete: new fields.NumberField({ initial: 0, integer: true }),
-      quantite: new fields.NumberField({ initial: 1, integer: true }),
-      equipped: new fields.BooleanField({ initial: false })
+      ...super.defineSchema()
     };
   }
 }