Silent payments page improvements

[yashrajd]

Creating this issue for new & existing (pending comments from PR improvements to silent payments page:

• add relative URL (sorry I totally missed this when checking against checklist PR)
• improve text legibility in mobile version of illustrations
• consider a modal in send page mockup to handle address delta where the diff might be hard to read
• fix header-1200x630 image for name repetition

Please assign this issue to me, I can tackle these and any (within reason) other items that come up after the page is public...