feat(ui): landing content sections, footer, use cases, and loading states [UI #13, #14, #15, #16]

[JoeX17]

Summary

Completes the landing page content and adds loading/copy utilities to the app layer.

Changes

UI #13 — HowItWorks, WhatWeDecode, and OpenSource sections

• src/components/landing/HowItWorksSection.tsx
• src/components/landing/WhatWeDecodeSection.tsx — supported operation types + coming-soon
• src/components/landing/OpenSourceSection.tsx — CTA linking to GitHub repo
• All three wired into src/app/page.tsx

UI #14 — Footer

• src/components/landing/Footer.tsx — project name, tagline, GitHub and Open App links
• Wired as last section in src/app/page.tsx

UI #15 — Use Cases section

• src/components/landing/UseCasesSection.tsx — animated node graph with IntersectionObserver
• Central "Stellar Explain" node, 5 audience nodes, curved lines with pulse animation
• Triggered on scroll into view, wired between WhatWeDecode and OpenSource

UI #16 — Skeleton loading states and copy utility

• src/hooks/useCopyToClipboard.ts — returns { copy, copied, copiedText } with reset delay
• src/components/Toast.tsx — fixed bottom-right notification, pop-in/out animation
• src/app/tx/[hash]/page.tsx — replace loading text placeholder with TransactionSkeleton
• src/app/account/[address]/page.tsx — replace loading text placeholder with AccountSkeleton
• src/app/app/page.tsx — wire useCopyToClipboard and Toast

How to test

npm run dev

1. Visit http://localhost:3000 — all landing sections visible in order
2. Scroll to Use Cases — node graph animates in
3. Visit http://localhost:3000/tx/[hash] — skeleton pulses while loading
4. Visit http://localhost:3000/account/[address] — skeleton pulses while loading
5. Any copy action triggers bottom-right toast for 2 seconds

Closes

Closes #161
Closes #162
Closes #163
Closes #164

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