fix(web): 404 pages keep the site's default title #877

Found in a live audit of rate.vn (round 4). In a browser, these pages return 404 but keep a wrong title, so the tab, history and screen readers do not say the page was not found (WCAG 2.2 SC 2.4.2):

• Unknown paths such as /khong-ton-tai-abc or /leaderboard/x ([locale]/[...rest]) show the site default, "Rate.vn - Giá thật, thông số và đánh giá đã lọc".
• /compare/a-vs-b shows "Đối đầu 1v1 | Rate.vn".

Missing products (/smartphones|laptops|earbuds/<unknown>) and merchants are already titled "Không tìm thấy sản phẩm/gian hàng | Rate.vn". Next streams page metadata after load, so a plain curl always shows the default title there; that is expected.

Note: generateMetadata in [locale]/not-found.tsx is not usable here. It made notFound() from pages answer 500 in a local build.

Acceptance: unknown paths and missing comparisons are titled "Không tìm thấy trang | Rate.vn" (localized). E2E covers vi and en.