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>

lib/cookies.ts

@@ -1,19 +0,0 @@
-import { cookies } from 'next/headers'
-
-const LAST_PRODUCT_COOKIE = 'lastProductId'
-const THIRTY_DAYS_SECONDS = 60 * 60 * 24 * 30
-
-export async function setLastProductCookie(productId: string) {
-  const store = await cookies()
-  store.set(LAST_PRODUCT_COOKIE, productId, {
-    httpOnly: true,
-    sameSite: 'lax',
-    maxAge: THIRTY_DAYS_SECONDS,
-    path: '/',
-  })
-}
-
-export async function getLastProductCookie(): Promise<string | null> {
-  const store = await cookies()
-  return store.get(LAST_PRODUCT_COOKIE)?.value ?? null
-}