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/actor-sheet.hbs

@@ -273,11 +273,7 @@
                   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>
 
@@ -336,7 +332,7 @@
             <li class="item flexrow " data-item-id="{{don._id}}" data-item-type="don">
               <img class="item-name-img" src="{{don.img}}" />
               <span class="item-name-label competence-name">{{don.name}}</span>
-              <span class="item-field-label-short">{{don.system.allegeance}}</span>
+              <span class="item-field-label-short">{{localizeAllegiance don.system.allegeance}}</span>
               <div class="item-filler">&nbsp;</div>
               <div class="item-controls item-controls-fixed">
                 <a class="item-control" data-action="editItem" title="Edit Item"><i class="fas fa-edit"></i></a>
@@ -365,7 +361,7 @@
             <li class="item flexrow " data-item-id="{{pacte._id}}" data-item-type="pacte">
               <img class="item-name-img" src="{{pacte.img}}" />
               <span class="item-name-label competence-name">{{pacte.name}}</span>
-              <span class="item-field-label-short">{{pacte.system.allegeance}}</span>
+              <span class="item-field-label-short">{{localizeAllegiance pacte.system.allegeance}}</span>
               <div class="item-filler">&nbsp;</div>
               <div class="item-controls item-controls-fixed">
                 <a class="item-control" data-action="editItem" title="Edit Item"><i class="fas fa-edit"></i></a>