[DwC export]: Add Export Package creation/edit form

[grantfitzsimmons]

Goal

Implement the Export Package form dialog with all required fields for creating and editing exportdataset records.

Form Fields

Field	Type	Validation
Export Name	Text	Required, unique
Core Mapping	Query combo box	Required, Core mappings only
File Name	Text	Required, ends with .zip, unique
Metadata	Query combo box	Required, EML app resource
Extensions	Subview (one-to-many)	Extension mappings only
Collection	Query combo box	Required
Include in RSS feed?	Checkbox	—
Frequency	Integer	Required if RSS checked, > 0

Acceptance Criteria

• All fields above are present and correctly typed in the form.
• ExportName and FileName uniqueness is enforced on save.
• FileName is validated to end with .zip.
• The Extensions subview allows adding/removing extension mappings.
• Saving creates or updates the exportdataset record.
• Form is accessible from both New (blank) and Edit (pre-filled) flows.