[FE-57] Create shipment proof of delivery upload flow

[mftee]

Overview

Build a flow for carriers to upload proof of delivery (photo + signature) when marking a shipment as delivered. All work goes in frontend/sandbox/.

Acceptance Criteria

• Component at frontend/sandbox/components/ProofOfDelivery.tsx
• Step 1: Upload delivery photo (camera capture or file upload)
• Step 2: Capture recipient signature using a canvas-based signature pad
• Step 3: Review and submit
• Preview of uploaded photo and drawn signature before submission
• Clear signature button on the signature pad
• onSubmit({ photoUrl, signatureDataUrl }) callback
• Demo page at frontend/sandbox/deliveries/[id]/page.tsx renders the flow