### What?
Improves both the JSON preview and export functionality in the
import-export plugin:
- Preserves proper nesting of object and array fields (e.g., groups,
tabs, arrays)
- Excludes any fields explicitly marked as `disabled` via
`custom.plugin-import-export`
- Ensures downloaded files use proper JSON formatting when `format` is
`json` (no CSV-style flattening)
### Why?
Previously:
- The JSON preview flattened all fields to a single level and included
disabled fields.
- Exported files with `format: json` were still CSV-style data encoded
as `.json`, rather than real JSON.
### How?
- Refactored `/preview-data` JSON handling to preserve original document
shape.
- Applied `removeDisabledFields` to clean nested fields using
dot-notation paths.
- Updated `createExport` to skip `flattenObject` for JSON formats, using
a nested JSON filter instead.
- Fixed streaming and buffered export paths to output valid JSON arrays
when `format` is `json`.
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