Create an original square Japanese summer coupon poster in orange, cream, and warm yellow, with a bold central discount panel and compact date strip. Text: “夏のお買い物応援フェア”, “最大20% 割引クーポン”, “期間限定 2026.08.01-2026.08.18”. Use only subtle sunburst lines and two small sparkle accents. Keep all visible text in Japanese. No brand, watermark, QR code, website, English text, Chinese text, or random tiny text.