Pre-fill completion proof when resubmitting a milestone

[damianmarti]

• I think it would be nice to show the completion_proof field in the CompleteMilestoneModal when the user resubmits. Probably they just need to tweak the field with something that was wrong/missing.

Cursor proposes something like:

const { formMethods, getCommonOptions } = useFormMethods<CompleteMilestoneModalFormValues>({
  schema: completeMilestoneModalFormSchema,
  defaultValues: {
    completionProof: milestone.completionProof || "",
  },
});

Originally posted by @Pabl0cks in #98 (review)