feat: sort-alpha button per group, settings in header, sortPosition pin

- ⇅ button on each group header: sorts units alpha (labelPlural + symbol tiebreak) via order API
- ⚙ Settings moved to admin-header alongside Log out
- Unit type gets sortPosition?: number (1-based, optional)
- Converter sort: partitions pinned/floating, inserts pins at requested slots
  Pins > length append at end; gaps fill naturally with floating items
- Admin unit form: Force position number input

data/units.json

@@ -7,7 +7,7 @@
       "symbol": "dl",
       "group": "dickloads",
       "toBase": 1,
-      "description": "See {pound:description}. Let's also look at {us-oil-bbl}."
+      "description": "The basic unit of measurement for all weights and masses worldwide."
     },
     {
       "id": "pound",
@@ -16,25 +16,26 @@
       "symbol": "lb",
       "group": "dickloads",
       "toBase": 0.038,
-      "description": "My description is {dickload:description}"
+      "description": "Avoirdupois pounds, as used in the USA."
     },
     {
       "id": "us-oil-bbl",
-      "label": "oil barrel (US)",
-      "labelPlural": "oil barrels (US)",
-      "symbol": "us oil bbl",
+      "label": "barrel",
+      "labelPlural": "barrels",
+      "symbol": "bbl us oil",
       "group": "dickloads",
       "toBase": 13.3192584,
       "description": "Weight is based on pure water at 4 degrees celsius.",
       "hidden": false
     },
     {
-      "id": "hogshead",
+      "id": "imperial-hogshead",
       "label": "hogshead",
       "labelPlural": "hogsheads",
-      "symbol": "hhd",
+      "symbol": "hhd imperial",
       "group": "dickloads",
-      "toBase": 38
+      "toBase": 27.420633296,
+      "description": "The hogshead as understood by residents of Old Blighty."
     },
     {
       "id": "short-ton",
@@ -108,26 +109,34 @@
       "id": "us-fluid-bbl",
       "label": "barrel",
       "labelPlural": "barrels",
-      "symbol": "us fluid bbl",
+      "symbol": "bbl us fluid",
       "group": "dickloads",
       "toBase": 9.9894438
     },
     {
       "id": "us-dry-bbl",
-      "label": "dry barrel",
-      "labelPlural": "dry barrels",
-      "symbol": "us dry bbl",
+      "label": "barrel",
+      "labelPlural": "barrels",
+      "symbol": "bbl us dry",
       "group": "dickloads",
-      "toBase": 1
+      "toBase": 9.6864954123
     },
     {
       "id": "us-hogshead",
-      "label": "hogshead (US)",
-      "labelPlural": "hogsheads (US)",
-      "symbol": "us hhd",
+      "label": "hogshead",
+      "labelPlural": "hogsheads",
+      "symbol": "hhd us",
       "group": "dickloads",
       "toBase": 19.9788876,
       "description": "Equal to two US fluid barrels."
+    },
+    {
+      "id": "imperial-barrel",
+      "label": "barrel",
+      "labelPlural": "barrels",
+      "symbol": "bbl imperial",
+      "group": "dickloads",
+      "toBase": 13.710316648
     }
   ],
   "groups": [