Add searchable taxonomy-based dropdown for domains

**Is your feature request related to a problem? Please describe.**
The Domains field in the Mentorship Program and Module forms is currently a free-text input. Users can enter any value (for example `AI`, `Artificial Intelligence`, `Web Dev`), which leads to inconsistent and duplicate domain names. This makes filtering and searching by domain unreliable.

**Describe the solution you'd like**
Change the Domains field to a searchable dropdown with autocomplete, using a predefined list of available domains (taxonomy). Users should be able to type and select a domain from this list instead of entering free text.

**Are you going to work on implementing this?**

- [x] Yes
- [ ] No

Ref #2701

Start at the Domains fields in the Mentorship Program and Module forms and inspect how their current free-text values are stored and filtered. Identify the available domain taxonomy and the existing form components, then verify that users can search for and select only taxonomy values and that domain filtering remains consistent.

Written by the indexing model from the issue text.