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)

src/lib/description.ts

@@ -0,0 +1,52 @@
+import type { Unit, Group } from './types';
+
+/**
+ * Resolves placeholder tokens in a description string.
+ *
+ * Supported formats (using the unit/group's ID):
+ *   {id}         → "Label (symbol)"          e.g. "Shit Ton (shttn)"
+ *   {id:label}   → singular label            e.g. "Shit Ton"
+ *   {id:plural}  → plural label              e.g. "Shit Tons"
+ *   {id:symbol}  → symbol                    e.g. "shttn"
+ *   {id:glabel}  → group label               e.g. "Mass"
+ *
+ * Unknown IDs or fields are left as-is.
+ */
+export function resolveDescription(
+  description: string | undefined,
+  units: Unit[],
+  groups: Group[]
+): string {
+  if (!description) return '';
+
+  const unitMap = new Map(units.map((u) => [u.id, u]));
+  const groupMap = new Map(groups.map((g) => [g.id, g]));
+
+  return description.replace(/\{([^}]+)\}/g, (match, token: string) => {
+    const colonIdx = token.indexOf(':');
+    const id = colonIdx === -1 ? token : token.slice(0, colonIdx);
+    const field = colonIdx === -1 ? '' : token.slice(colonIdx + 1);
+
+    const unit = unitMap.get(id);
+    if (unit) {
+      switch (field) {
+        case '':       return `${unit.label} (${unit.symbol})`;
+        case 'label':  return unit.label;
+        case 'plural': return unit.labelPlural;
+        case 'symbol': return unit.symbol;
+        default:       return match; // unknown field — leave as-is
+      }
+    }
+
+    const group = groupMap.get(id);
+    if (group) {
+      switch (field) {
+        case '':       return group.label;
+        case 'label':  return group.label;
+        default:       return match;
+      }
+    }
+
+    return match; // unknown ID — leave as-is
+  });
+}

src/lib/types.ts

@@ -27,6 +27,9 @@ export interface Unit {
 
   /** Optional human-readable description shown as a tooltip in the converter. */
   description?: string;
+
+  /** If true, this unit is excluded from automatic rendering in the converter. */
+  hidden?: boolean;
 }
 
 /** A group of units sharing a base unit and a universal scale factor. */
@@ -39,6 +42,8 @@ export interface Group {
   toUniversal: number;
   /** Display order for units in the converter. Defaults to 'defined'. */
   sortOrder?: 'defined' | 'alpha';
+  /** If true, this group (and all its units) is excluded from automatic rendering. */
+  hidden?: boolean;
 }
 
 /** Full data payload for units and groups. */