docs: update specs for schema-driven form descriptor API

- frontend-spec.md: rewrite CreateItemPane spec for dynamic form
  rendering from form descriptor, replace CategoryPicker three-column
  spec with multi-stage domain/subcategory picker, replace useCategories
  hook with useFormDescriptor, update form sections to dynamic field
  groups, mark hierarchical categories as implemented, remove
  sourcing_link/standard_cost from item-level DB columns, update types
  and implementation order
- SPECIFICATION.md: rename /api/schemas/{name}/properties endpoint to
  /api/schemas/{name}/form

docs/SPECIFICATION.md

@@ -627,7 +627,7 @@ POST   /api/uploads/presign                                 # Get presigned MinI
 # Schemas (read: viewer, write: editor)
 GET    /api/schemas                                         # List all schemas
 GET    /api/schemas/{name}                                  # Get schema details
-GET    /api/schemas/{name}/properties                       # Get property schema for category
+GET    /api/schemas/{name}/form                             # Get form descriptor (field groups, widgets, category picker)
 POST   /api/schemas/{name}/segments/{segment}/values        # Add enum value [editor]
 PUT    /api/schemas/{name}/segments/{segment}/values/{code} # Update enum value [editor]
 DELETE /api/schemas/{name}/segments/{segment}/values/{code} # Delete enum value [editor]