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-equipment.hbs

@@ -2,6 +2,8 @@
   <div class="slots-counter">
     <span class="slots-label">{{localize "OATHHAMMER.Label.ItemSlots"}}</span>
     <span class="slots-value {{#if slotsOver}}slots-over{{/if}}">{{slotsUsed}} / {{slotsMax}}</span>
+    <span class="slots-bonus-label">{{localize "OATHHAMMER.Label.SlotsBonus"}}</span>
+    <input type="number" name="system.inventory.slotsBonus" value="{{system.inventory.slotsBonus}}" class="slots-bonus-input">
   </div>
   <fieldset class="currency-bar">
     <legend>{{localize "OATHHAMMER.Label.Currency"}}</legend>