[DwC export]: Add "Term" mapping column to Schema Mapping interface

[grantfitzsimmons]

Goal

Extend the query builder–style schema mapping interface with an additional Term column allowing users to associate each output field with a schema concept (IRI or term name).

Background

The schema mapping interface is built on top of the Query Builder. The Term column is the key differentiator between a standard query and a schema mapping — it's what makes the output DwC-compliant.

Acceptance Criteria

• A Term column appears to the left of each field row in the schema mapping interface.
• The column renders as a searchable dropdown populated from the selected schema vocabulary's terms.
• Term selection stores the full IRI (e.g., http://rs.tdwg.org/dwc/terms/catalogNumber) in spqueryfield.Term (new table, see other issue)
• Rows without a term selected are still valid (the field exports but is not labeled as a DwC concept).

[CarolineDenis]

NOTES:

• File started in Feat: Add DwC section in Schema config #7867