Decouple the generic Dropdown from the root route

The generic Dropdown component (frontend/src/components/ui/Dropdown.tsx) imports Route from routes/__root and calls Route.useSearch() to read a genres param. A reusable UI component reading another page's search params is backwards, and on the stats page that param doesn't even exist.

• Dropdown receives its selection via props, whoever renders it reads the URL
• Remove the initialSync effect that mixes URL state with the defaultItem fallback
• Both usages (album genre filter, stats genre picker) keep working, driven by their own route's search params