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

src/lib/types.ts

@@ -30,6 +30,14 @@ export interface Unit {
 
   /** If true, this unit is excluded from automatic rendering in the converter. */
   hidden?: boolean;
+
+  /**
+   * Optional fixed display position (1-based) within the group's rendered list.
+   * Pinned units are inserted at their requested slot after floating units are ordered.
+   * If position > total units, the unit is appended at the end in ascending sortPosition order.
+   * Ties are broken by the group's natural sort order.
+   */
+  sortPosition?: number;
 }
 
 /** A group of units sharing a base unit and a universal scale factor. */