Add selectable EXIF ImageDescription fields for process and scanning metadata

[jboneng]

Summary

EXIF ImageDescription can now include process and scanning metadata as well as gear. A Description… button on the Metadata preview opens a checklist of which fields to join into the description, so JPEG workflows that search description text (export → DAM / Photos) can filter on developer, format, push/pull, and scanning without cluttering every file by default.

Behaviour

Checked fields with non-empty values are joined with • into ImageDescription. Push/pull is omitted when Normal.

Field	Default
Camera	On
Lens	On
Film stock	On
Film ISO	On
Format	Off
Developer	Off
Push / Pull	Off
Scanning	Off

Defaults match the previous gear-only description.

Per-frame and batch

• Selection is stored per frame on MetadataConfig.description_fields.
• New frames inherit the last used selection (last_description_fields sticky).
• Sync custom metadata to all files in batch export and Sync settings can copy it with the rest of metadata.
• Protect original metadata still skips NegPy metadata writes (including description).

Example (all fields enabled)

Canon AE-1 • 50mm f/1.4 • Portra 400 • ISO 400 • 35mm • D-76 1+1 • Push +1 • DSLR copy-stand

Test plan

• Export JPEG with defaults — description is still camera • lens • film • ISO only.
• Enable Format / Developer / Scanning via Description… — description and Metadata preview update; export includes those values.
• Push/Pull = Normal with that field enabled — Normal does not appear in the description.
• Change selection, open a new frame — sticky selection applies; a previously saved frame keeps its own selection.
• Sync metadata / batch with sync on — selection copies to other frames.
• Protect original metadata on — source description unchanged; Description… disabled.

Implements #705