[DwC export]: Allow entering a custom IRI in the Term column

[grantfitzsimmons]

Goal

Users must be able to type or paste a custom IRI into the Term column for concepts not present in the system terms JSON.

Background

New DwC terms or extension terms not yet in the JSON may still need to be mapped. Custom IRIs future-proof the system without requiring a software update.

Acceptance Criteria

• The Term column accepts free-text input in addition to dropdown selection.
• If a user types a value not present in the vocabulary list, it is accepted as-is (custom IRI).
• Custom IRIs are validated to be well-formed URLs (basic format check); a warning is shown for malformed values but saving is not blocked.
• Custom IRIs are stored in spqueryfield.Term identically to system IRIs.
• Custom IRIs do not propagate to other mappings' Term dropdowns (they are local to the mapping where entered).