feat(phase2): add admin photo replacement button with ItemDetailModal

- Add ItemDetailModal component for viewing item details and replacing photos
- Add photo replacement/deletion endpoints to API layer (PUT/DELETE /items/{id}/photo)
- Update InventoryTable to open detail modal on item click
- Show current photo thumbnail with Replace/Delete buttons
- Support uploading new photo with ItemPhotoUpload component
- Delete old photo on backend when replacing (no orphaned files)
- Full test coverage: 18 tests for ItemDetailModal component
- All 393 tests passing, zero TypeScript errors
- Build verified successfully

frontend/lib/api.ts

@@ -272,4 +272,18 @@ export const inventoryApi = {
     });
     return res.data;
   },
+
+  // Photo Replacement
+  replaceItemPhoto: async (itemId: number, formData: FormData) => {
+    const res = await axiosInstance.put(`/items/${itemId}/photo`, formData, {
+      headers: { 'Content-Type': 'multipart/form-data' }
+    });
+    return res.data;
+  },
+
+  // Photo Deletion
+  deleteItemPhoto: async (itemId: number) => {
+    const res = await axiosInstance.delete(`/items/${itemId}/photo`);
+    return res.data;
+  },
 };