Provide the "submission ID" to all users on the confirmation page

## Context from first instance

Currently the submission ID is provided to form fillers only if they download a copy of their answers (on the confirmation page), **when saving answers is ON**

To help us troubleshoot potential lost/missing submissions -- we might consider adding this to the confirmation page by default.

As per incident [#incident-2025-08-25-form-failing-to-submit-some-responses](https://gcdigital.slack.com/archives/C09BYDEQWJ2)

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## Context from second instance

This emerged again in our conversation with a team at Finance that was creating workaround scripts to be able to provide each form filler with their submission ID by email once they submitted a form --> #7418

This could be mitigated by providing the submission ID clearly on the confirmation page upon submit, this is already provided in the "copy of submission" that can be downloaded **if "saving answers" is ON**.

https://docs.google.com/document/d/1RIqI7UZ7jxOZcLd845q7IjaxiON7563mZXuFWeaTMC0/edit?tab=t.0

## Related

#6252