feat: add responsive image srcset on CreditLines campaign hero (Closes #848)

[waterWang]

Summary

Adds a responsive campaign hero image to the CreditLines page with proper srcSet and sizes attributes, so mobile devices download only the image resolution they need instead of desktop-sized assets.

Closes #848

Changes

src/pages/CreditLines.tsx

• Added a hero banner image section with srcSet containing 3 width candidates (480w, 768w, 1200w)
• Configured sizes attribute for responsive breakpoint-aware image selection
• Accessible alt text and role="img" container

src/pages/CreditLines.css

• Added .cl-hero-image container styles (border-radius, overflow, background)
• Added .cl-hero-img responsive image styles (100% width, auto height, max-height 240px)

src/pages/__tests__/CreditLines.responsive.test.tsx

• 4 new tests covering:
  • Renders campaign hero image with accessible alt text
  • Includes srcset attribute with 480w, 768w, 1200w candidates
  • Configures sizes attribute with 100vw and 60vw breakpoints
  • Provides default fallback src attribute

Acceptance Criteria

• Implement per the description above — srcset for images on CreditLines
• Add focused tests for the change
• Document any visible changes
• Responsive across all breakpoints
• WCAG 2.1 AA accessibility (alt text, role="img", aria-label)
• Design-token + dark-mode consistency
• Clear documentation and inline comments