Add top-level NFTs navigation

[lai3d]

Summary

• add a top-level NFTs navigation entry in desktop and mobile nav
• add a dedicated /nfts landing page for ERC-721 and ERC-1155 collections
• keep the existing Tokens menu focused on ERC-20 flows and add translation keys for the new NFT entry/page

Checks

• npm run typecheck: passed
• npm run build: passed
• npx vitest run --passWithNoTests: failed due to existing localStorage.clear test-environment issues in src/tests/lib/i18n.test.ts and src/tests/components/ThemeToggle.test.tsx