feat: add hero section to Landing page

[GreatShinro]

feat: Landing page hero section

Branch: feat/landing-hero → main

Summary

Implements the hero section for the Landing page with a clear value proposition, prominent CTA, and visual elements that communicate the blockchain/healthcare context of VacciChain.

Changes

• Headline — "Tamper-Proof Vaccination Records on the Blockchain" with accent colour on the blockchain phrase
• Subheadline — existing i18n landing.subtitle string (no new copy added)
• Primary CTA — "Connect Freighter Wallet" button triggers the wallet connection flow; toggles to a disconnect state when a wallet is already connected
• Secondary CTA — "Verify a Record" link routes to /verify for unauthenticated visitors
• Visual context — dark blue-to-purple gradient background; "Stellar Testnet" network badge; four feature pills (🔗 Stellar Blockchain, 🛡️ Soulbound NFTs, ✅ Instant Verification, 🏥 Issuer-Gated Minting)
• Responsive — clamp() font sizes, flexWrap on CTA row and feature list, minHeight: 100vh keeps content above the fold at 768px

Files Changed

File	Change
frontend/src/pages/Landing.jsx	Rewrote hero section

Acceptance Criteria

• Hero includes headline, subheadline, and primary CTA button
• CTA button links to wallet connection flow
• Hero is responsive on mobile and desktop
• Visual elements reinforce the blockchain/healthcare product concept
• Hero content is above the fold on a 768px viewport

Testing

1. Run cd frontend && npm install && npm run dev
2. Open http://localhost:3000
3. Verify hero is fully visible without scrolling at 768px viewport width
4. Click "Connect Freighter Wallet" — Freighter extension prompt should appear
5. After connecting, confirm wallet address is shown and "Disconnect" button appears
6. Click "Verify a Record" — should navigate to /verify
7. Toggle dark mode and confirm gradient and text remain legible

closes #289

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