The root layout called `await cookies()` to read the locale, which
marked the entire app as dynamic. In Next.js 16, dynamic pages have
Router Cache staleTime=0, causing a fresh RSC server roundtrip on
every navigation — the root cause of ~400ms tab switching delays.
- Remove cookies() from root layout, making it static
- Add LocaleSync client component to read locale cookie on the client
- Add loading.tsx skeleton for dashboard routes as a loading fallback
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