Onboarding backup import still buffers the entire ZIP in memory #944

Problem

Issue #555 moved backup export/import/sync to bounded-memory streaming, including the native import path. However, the onboarding backup restore path still bypasses that transport.

In lib/src/onboarding_feature/steps/import_step.dart, _onZipSelected() currently does:

final bytes = await File(filePath).readAsBytes();
...
request.add(bytes);

This loads the complete selected backup ZIP into memory before POSTing it to /api/v1/data/import.

By contrast, Settings > Data Management uses BackupTransferService.uploadZip(...), which streams the file and is aligned with #555.

This was noticed while triaging #918; it is not the cause of #918's too_many_entries rejection, but it is an inconsistency with the bounded-memory guarantees introduced by #555.

Expected behavior

All native Decaid backup-import entry points should use the same bounded-memory/streaming transport. Onboarding should not materialize a complete backup archive in a List<int> / Uint8List.

Suggested implementation

• Reuse BackupTransferService.uploadZip(...) from the onboarding ZIP-import path instead of manually reading the file and constructing an HttpClient request.
• Preserve current onboarding behavior:
  • POST to /api/v1/data/import?onConflict=skip;
  • convert the response through BackupImportResponse;
  • call persistenceController.notifyShotsChanged();
  • display the existing ImportResultView.
• Keep error handling consistent with the Settings import path where practical.
• Do not change the server-side archive limits or ZIP validation as part of this issue.

Acceptance criteria

• Onboarding ZIP restore no longer calls File.readAsBytes() for the backup archive.
• The selected ZIP is streamed/file-backed end-to-end using the existing backup transfer abstraction.
• Existing conflict semantics remain onConflict=skip.
• Existing successful and failed import UI behavior is preserved.
• Add focused coverage proving the onboarding path does not regress to whole-file buffering.
• flutter analyze and relevant tests pass.

Context

• Follow-up to Stream backup export, import, and sync data instead of fully buffering it #555.
• Discovered while triaging Improve UX when a non-Decaid ZIP is rejected during backup import #918.