perf(export): streaming chunked JSON export without intermediate Map and list allocations #1535

Problem

In DataExportService.formatJson (lib/shared/services/data_export_service.dart), exporting table data creates heavy memory pressure:

1. Allocates a new Map<String, Object?> for every row (100,000 maps for 100,000 rows).
2. Collects all maps into a list of 100,000 items.
3. Encodes the whole structure via JsonEncoder.convert, creating a single massive string in RAM before writing to disk, leading to potential OOM on large datasets.

Solution

• Stream JSON chunks directly into StringSink / IOSink using zero-allocation chunked formatting:
  buffer.write('{\n "'), buffer.write(col), buffer.write('": '), _writeJsonValue(buffer, val).
• Eliminate all intermediate Map objects and the outer list.

Benefits

• 0 intermediate Map allocations during JSON export.
• Flat memory footprint regardless of row count, avoiding Out-Of-Memory errors on large exports.