Essentially, my next move would be to submit the proposal. Even though it isn't under my jurisdiction to have direct contact with potential clients, my boss has clearly entrusted me to review and analyze the proposal without any specific guidance. I would check for potential errors, look for key criterion, and finally submit to the portal/designated spot for submission.
I would then immediately email my boss and explain the situation at hand. This is my chosen point of action because any proposal is better than no proposal and potentially losing a client. As no one is left in the office, I have to assume leadership and take initiative as my internship at this company is supposed to uphold and uplift the community as a whole.
I have to trust in my powers that I can effectively look over and finish off projects as I have worked with my team and boss for majority of the proposal. The final sentence further strengthens my point as the key phrase "you are the only person in the office who can make decisions on the proposal" indicate I presume responsibility when it comes to this resolution.