Move odometer image stitching to confirmation page

[Julesssss]

After #83556, the odometer stitching logic runs on the odometer step and blocks navigation until complete. Per design feedback, this should instead:

1. Navigate to the confirmation page immediately
2. Run stitching in the background, showing a loading indicator on the receipt preview
3. Handle stitching failure gracefully on the confirmation page

This improves perceived performance by letting users edit other expense details while the image stitches.

Issue Owner

Current Issue Owner: @DylanDylann

[melvin-bot]

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If you are the assigned CME please investigate whether the linked PR caused a regression and leave a comment with the results.

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[melvin-bot]

⚠️ Looks like this issue was linked to a Deploy Blocker here

If you are the assigned CME please investigate whether the linked PR caused a regression and leave a comment with the results.

If a regression has occurred and you are the assigned CM follow the instructions here.

If this regression could have been avoided please consider also proposing a recommendation to the PR checklist so that we can avoid it in the future.