🎨 Palette: UX polish and accessibility for V2CouponCenter

[godie]

This PR improves the UX and accessibility of the V2CouponCenter page.

💡 What:

• Integrated Util.showToast to provide visual feedback when a user copies a coupon code.
• Replaced the manual preloader with the reusable CustomPreloader component.
• Added aria-label="Volver" to the header back button.
• Added aria-hidden="true" to all decorative Material Icons.
• Updated button classes from v2-btn-primary (undefined) to v2-btn-filled.

🎯 Why:

• Users need immediate feedback after interacting with "Copy to clipboard" actions.
• Standardizing components improves maintainability and visual consistency.
• Proper ARIA attributes ensure the interface is usable by screen reader users.

♿ Accessibility:

• Back button now has a descriptive label.
• Decorative icons are hidden from screen readers to reduce noise.
• Loading state uses semantic role="progressbar".

This PR also addresses pre-existing linter errors in V2CaseStudy.jsx (unused import) and V2FlashcardStudy.jsx (unused variable) to ensure a clean build.

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PR created automatically by Jules for task 10686080230564546550 started by @godie

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