Sync landing nav and footer links

[huazhuang80-star]

Summary

• Adds a shared landing section link config in lib/landing-nav.ts.
• Uses that shared source for the desktop/mobile navbar and footer Links column.
• Adds footer access to the major landing sections without crowding the header nav.

Related issue

Closes #11

Type of change

• refactor: shared navigation config
• feat: additional landing section links in the footer

Test plan

• Production build passes via node node_modules/next/dist/bin/next build
• Confirmed mobile menu still uses the same link click close handler

Notes for reviewers

The header intentionally keeps a concise subset of primary links, while the footer lists the broader landing sections from the same source.

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