feat: hidden flag, description placeholders, drag handle contrast fix

- Unit and Group get hidden?: boolean — filtered server-side before render
- Hidden group also suppresses all its units from the converter
- New resolveDescription() utility in src/lib/description.ts
  supports {id}, {id:label}, {id:plural}, {id:symbol} placeholders
- ConverterCard pre-resolves descriptions and passes result to ConversionResult
- ConversionResult accepts description prop instead of reading unit.description
- Admin forms: Hidden checkbox added for both units and groups
- drag-handle color overridden to --pico-primary-inverse on selected/drag-over rows
  (white in light/dark themes, black on Dan Mode yellow)

data/units.json

@@ -58,7 +58,7 @@
     },
     {
       "id": "fuckton",
-      "label": "Fuckton",
+      "label": "Fuck ton",
       "labelPlural": "Fucktons",
       "symbol": "fcktn",
       "group": "dickloads",
@@ -86,7 +86,7 @@
       "labelPlural": "Shit tons",
       "symbol": "shttn",
       "group": "dickloads",
-      "toBase": 25.3333333333
+      "toBase": 38
     }
   ],
   "groups": [