[DwC export]: Add vocabulary/term list selection dialog

[grantfitzsimmons]

Goal

When a user chooses Create From Scratch in the new Schema Mapping dialog, present a dialog to select the schema vocabulary whose terms will populate the Term dropdown in the mapping interface.

Background

The term pick list in the schema mapping interface must be scoped to the vocabulary the user is mapping to (e.g., only Darwin Core terms when building a DwC Occurrence mapping).

Acceptance Criteria

• The vocabulary selection dialog lists all vocabularies present in the schema terms JSON (e.g., dwc (Darwin Core), ac (Audiovisual Core) with friendly names.
• Each vocabulary entry shows its display name, abbreviation, and vocabularyURI.
• After selection, the schema mapping interface opens with its Term dropdown restricted to terms from the chosen vocabulary.
• Once a vocabulary is set for a schema mapping, it cannot be changed.

[CarolineDenis]

NOTES:

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