[FRONTEND] Merch page Showcase intro section

[yrska]

🎯 Acceptance Criteria

• Implements the "SHOWCASE" heading with the curved arch background shape behind it.
• Renders the descriptive paragraph text beneath the heading.
• Uses the correct font, colour, and spacing from the design system.
• Tests for responsiveness across mobile/tablet/desktop.

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📊 Definition of Done (DoD)

• Acceptance criteria met
• Agreed tests written
• Agreed documentation in GitHub wiki
• PR reviewed

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🔗 Dependencies

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📚 Additional Context