fix(frontend): say the database is missing, not Supabase

Self-hosted PostgreSQL is the runtime. A missing APP_DATABASE_URL
must not tell users the retired hosted service is unconfigured.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/components/email-otp-login.tsx

@@ -19,7 +19,7 @@ function authMessage(caught: unknown) {
     message.includes("environment") ||
     message.includes("URL")
   )
-    return "Supabase 尚未配置";
+    return "数据库尚未配置";
   if (lower.includes("expired") || lower.includes("invalid"))
     return "验证码错误或已过期，请重新获取";
   if (lower.includes("rate limit")) return "发送过于频繁，请稍后再试";