fix: certificate PDF layout, partner logo capture, and download size

[choeying-10]

Summary

• Fix downloaded certificate PDFs showing white gaps top/bottom by sizing the jsPDF page to the screenshot instead of forcing A4 letterboxing.
• Fix missing course provider (partner) logo in the PDF by inlining the logo as base64 so html2canvas can paint it without CORS issues; wait for images before capture and tighten capture-mode CSS so the orange panel stays flush.
• Reduce download size from ~15MB to ~1.2MB by embedding JPEG (quality 1) at scale 2 instead of lossless PNG.

#Note: checked the prod certificate and the result is same.

[yogeshbhagatcode]

I hope this was tested and compared with current production certificate preview & print functionality.