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/mournblade-cyd2-actor.js

@@ -634,9 +634,7 @@ export class MournbladeCYD2Actor extends Actor {
     let valueSC = 0
     for (let monnaie of this.items) {
       if (monnaie.type == "monnaie") {
-        valueSC += Number(monnaie.system.prixsc) * Number(monnaie.system.quantite)
-        valueSC += (Number(monnaie.system.prixca) * Number(monnaie.system.quantite)) * 10
-        valueSC += (Number(monnaie.system.prixpo) * Number(monnaie.system.quantite)) * 100
+        valueSC += MournbladeCYD2Utility.getItemValueSC(monnaie)
       }
     }
     return MournbladeCYD2Utility.computeMonnaieDetails(valueSC)
@@ -647,9 +645,7 @@ export class MournbladeCYD2Actor extends Actor {
     let valueSC = 0
     for (let equip of this.items) {
       if (equip.type == "equipement" || equip.type == "arme" || equip.type == "protection") {
-        valueSC += Number(equip.system.prixsc) * Number(equip.system.quantite ?? 1)
-        valueSC += (Number(equip.system.prixca) * Number(equip.system.quantite ?? 1)) * 10
-        valueSC += (Number(equip.system.prixpo) * Number(equip.system.quantite ?? 1)) * 100
+        valueSC += MournbladeCYD2Utility.getItemValueSC(equip)
       }
     }
     return MournbladeCYD2Utility.computeMonnaieDetails(valueSC)