feat: sync landing nav and footer links

[silentgeckoaudit3801]

Summary

• add a shared lib/landing-nav.ts source for landing-section anchors
• update the desktop/mobile navbar to consume the shared header link subset
• update the footer Links column to consume the same source and expose the remaining major sections

Validation

• Static source review only: no dependencies were installed and no local build was run in this environment.

Closes #11

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