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)

less/item-list.less

@@ -302,6 +302,7 @@
 // Slots counter on the Combat tab
 .slots-counter {
   display: flex;
+  align-items: center;
   gap: 6px;
   padding: 2px 6px 4px;
 
@@ -330,6 +331,22 @@
       border-color: fade(#c0392b, 40%);
     }
   }
+
+  .slots-bonus-label {
+    font-size: @font-size-xs;
+    color: fade(@color-dark, 60%);
+    margin-left: 6px;
+  }
+
+  .slots-bonus-input {
+    width: 3rem;
+    font-size: @font-size-xs;
+    text-align: center;
+    padding: 1px 4px;
+    border: 1px solid fade(@color-gold, 40%);
+    border-radius: 4px;
+    background: fade(@color-gold, 8%);
+  }
 }
 
 // ── Regiment list (settlement garrison tab) ──────────────────────────────────