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/miracle-cast-dialog.hbs

@@ -32,6 +32,7 @@
     <div class="pool-info-line">
       {{localize "OATHHAMMER.Skill.Magic"}} ({{localize "OATHHAMMER.Attribute.Willpower"}} {{wpRank}})
       + {{localize "OATHHAMMER.Label.SkillRank"}} {{magicRank}}
+      {{#if magicModDisplay}} {{localize "OATHHAMMER.Label.SkillModifier"}} {{magicModDisplay}}{{/if}}
       = <strong>{{basePool}}d6</strong>
     </div>