fix: visually block off-hand slot when 2H weapon equipped in main hand

- Add getTwoHandedBlocker() helper to EquipmentSlotGrid that detects when
  mainHand has a two-handed weapon and returns the weapon name
- Render offHand slot with Lock icon + 'Blocked: <weapon name>' label +
  reduced opacity when blocked, distinct from normal 'Empty' dashed-border slot
- Add regression test verifying all 4 two-handed types are correctly flagged
  and non-two-handed types remain unblocked (11 tests)

docs/project-structure.txt

@@ -96,6 +96,7 @@ Mana-Loop/
 │   │   │   │   │   └── SpireDebugSection.tsx
 │   │   │   │   ├── EquipmentTab/
 │   │   │   │   │   ├── EquipmentEffectsSummary.tsx
+│   │   │   │   │   ├── EquipmentSlotGrid.test.ts
 │   │   │   │   │   ├── EquipmentSlotGrid.tsx
 │   │   │   │   │   └── InventoryList.tsx
 │   │   │   │   ├── SpireCombatPage/