feat(ST-903): load active product in layout, replace cookie with DB lookup in solo

- layout.tsx: fetch active_product_id, resolve product, clear stale ref server-side
- NavBar: add activeProduct prop (rendering changes in ST-904)
- solo/page.tsx: redirect via user.active_product_id instead of lastProductId cookie
- proxy.ts: remove lastProductId cookie logic
- lib/cookies.ts: deleted (no longer used)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/shared/nav-bar.tsx

@@ -15,9 +15,10 @@ interface NavBarProps {
   userId: string
   username: string
   email: string | null
+  activeProduct: { id: string; name: string } | null
 }
 
-export function NavBar({ isDemo, roles, userId, username, email }: NavBarProps) {
+export function NavBar({ isDemo, roles, userId, username, email, activeProduct: _activeProduct }: NavBarProps) {
   const pathname = usePathname()
   const currentProduct = useProductStore(s => s.currentProduct)