feat: editable unit/group IDs with full cascade

- Unit ID rename: updates group.baseUnitId refs + {old-id:*} in all descriptions
- Group ID rename: updates unit.group refs + {old-id:*} in all descriptions
- Admin form ID fields unlocked; hint text updated
- renameIdInDescription() shared helper in both PUT handlers

data/units.json

@@ -24,7 +24,9 @@
       "labelPlural": "barrels",
       "symbol": "bbl",
       "group": "dickloads",
-      "toBase": 19
+      "toBase": 13.319,
+      "description": "Weight is based on pure water at 4 degrees celsius.",
+      "hidden": false
     },
     {
       "id": "hogshead",
@@ -101,6 +103,14 @@
       "group": "another-group",
       "toBase": 1,
       "hidden": true
+    },
+    {
+      "id": "us-fluid-bbl",
+      "label": "US fluid barrel",
+      "labelPlural": "US fluid barrels",
+      "symbol": "us fluid bbl",
+      "group": "dickloads",
+      "toBase": 9.9894438
     }
   ],
   "groups": [