feat: luck bonus, inventory slots bonus, multi-enhancements, magic skill modifier

- Add luck.bonus field to character DataModel; included in prepareDerivedData
   (luck.max = fate.rank + bonus); shown as editable +bonus field next to
   luck max in edit mode (same UX as grit bonus)

 - Add inventory.slotsBonus field to character DataModel; Equipment tab now
   shows an editable "Bonus Slots" input that adds to the calculated max slots
   (10 + Might×2 + bonus)

 - Replace single Enhancement <select> in spell cast dialog with a checkbox
   list; multiple enhancements can now be selected simultaneously — stress
   costs, pool penalties, and boolean flags (redDice, noStress) are aggregated
   across all active enhancements

 - Include skills.magic.modifier in basePool for both spell and miracle dialogs
   and in baseDice in rollSpellCast / rollMiracleCast; modifier is shown in
   the dialog pool-info line and in the chat card when non-zero

 - Fix: pool-reduction indicator in rollSpellCast now compares against
   intRank + magicRank + magicMod (was missing magicMod)

templates/actor/character-sheet.hbs

@@ -72,7 +72,13 @@
                 <a data-action="adjustLuck" data-delta="1" class="luck-btn">+</a>
               </div>
               <span class="res-sep">/</span>
-              {{formInput systemFields.luck.fields.max value=system.luck.max name="system.luck.max" disabled=isPlayMode}}
+              <span class="luck-max-group">
+                {{formInput systemFields.luck.fields.max value=system.luck.max name="system.luck.max" disabled=true}}
+                {{#unless isPlayMode}}
+                  <span class="res-bonus-label">+</span>
+                  {{formInput systemFields.luck.fields.bonus value=system.luck.bonus name="system.luck.bonus" class="res-bonus-input"}}
+                {{/unless}}
+              </span>
             </div>
             <div class="character-resource">
               <span class="resource-label">{{localize "OATHHAMMER.Label.Movement"}}</span>