Add unit property to NumberRange component

[jeanaurent]

Problem:
The current NumberRange component accepts raw numeric inputs without context. Users entering values like "1000-5000" do not indicate the measurement unit (kilograms, meters, dollars, etc.), leading to ambiguity in data entry and poor user experience.

Solution:
Show unit labels (e.g., "kg", "%", "m", "USD") as prefixes to the min and max input fields.

Description of the proposal:

1. The backend must specify the unit code as a string
2. Add unit property to NumberRange Class

class NumberRange {
  ...
  public Unit: string | null = null; 
}

and to the controller

export class Controller extends InputController<NumberRange> {
  ...
  public unit: string | null = null;
}

3. Implement a prefix field to each input by using the "input-group-text" class

• Displayed only if the unit value is not null.
• Cannot be edited
• Example of implementation:

{#if unit}
  <span class="input-group-text">{unit}</span>
{/if}

4. No serialisation changes in the NumberRange component