Footer: add CoreTrustSeal logo with responsive right-side alignment

[shiftmed]

Adds the CoreTrustSeal logo to the footer and keeps layout stable across screen sizes.

• Added core-trust-seal-logo.png under the social section, linking to https://www.coretrustseal.org/.
• Reworked footer bottom row into a links+seal layout so policy links stay on the left and the seal stays on the right.
• Matched seal right margin to the social icons’ right margin using a shared CSS variable.
• Kept the seal responsive and vertically aligned with footer links.
• Ensured small screens keep the seal to the right of links (no stacking below).

Files Changed

• angular/src/app/frame/footbar/footbar.component.html
• angular/src/app/frame/footbar/footbar.component.css
• angular/src/assets/images/core-trust-seal-logo.png

Validation

• Verified on desktop: seal appears below social icons at footer right; links remain to its left.
• Verified on small screens: seal remains right-aligned and does not drop below link rows.

[RayPlante]

My initial review revealed two issues with the footer after including the CTS logo:

• the footer links ("Site Privacy", etc.) were now left-justified rather than centered
• A hard line-break before "Commerce.gov" was causing some inelegant line wrapping

I asked @chuanlin2018 to address these in Mehdi's absence; now looks good (thanks, Chuan!).