Replace placeholder icons with recolored unDraw scenes

[MustafaGood]

Summary

• Fills the large illustration slots (how-it-works, create-account, authorities search) with unDraw scenes, recolored from #6c63ff to brand accent #E6722A.
• USP row icons stay as they are; those are small UI icons and belong with the open-icons work, not this PR.
• Create-account uses the booklet illustration gradient (#20384E to #000000). Scenes are decorative (empty alt, wrapper aria-hidden).

Test plan

• Homepage SV + EN: three how-it-works scenes load (not Material icons)
• /profile/sign_in: sign-up scene on the dark illustration panel, desktop and mobile
• /body EN: file-searching scene next to the authorities intro
• Mobile 390px: scenes stack above the step copy, no overflow