docs: update navigation structure in ui-design.md

Remove dropdown menu reference and update navigation documentation to reflect
the simplified header navigation (Articles, Projects, Members only).

Author: aster-void

docs/knowledges/ui-design.md

@@ -41,7 +41,6 @@
 | Section labels      | Monospace, uppercase, tracking-widest, text-[#00D372]                                    |
 | Tech tags           | Monospace, text-xs, bg-zinc-100/zinc-800, rounded-lg                                     |
 | Page headers        | border-b, bg-zinc-50/50, py-16                                                           |
-| Dropdown menus      | dropdown-hover, border border-zinc-200, bg-white, shadow, hover:bg-primary/5             |
 
 ## Interaction Patterns