fix: prevent chat source citation clipping

[vibhor-aggr]

Summary

Closes #2408.

Prevents AI answer source/citation cards from being clipped horizontally by:

• allowing the source list to show a thin horizontal scrollbar instead of hiding the scroll affordance
• preventing individual source cards from shrinking in the flex row
• reserving right-side padding so the final card is not obscured by the fade overlay
• allowing long citation text or URLs to wrap inside the card

Verification

• npx prettier --check src/components/patterns/Searchbox/ChatSources.css
• npm run lint -- src/components/patterns/Searchbox/ChatSources.css (exits 0; CSS is ignored by the current ESLint config)
• git diff --check
• npm run build

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