/* ===== 全局重置 ===== */
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: 'Segoe UI', 'Helvetica Neue', Arial, sans-serif;
}

body {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  background: linear-gradient(135deg, #1e1e2f 0%, #2d2d44 100%);
  padding: 20px;
}

/* ===== 计算器容器 ===== */
.calculator {
  width: 100%;
  max-width: 360px;
  background: #2a2a3d;
  border-radius: 20px;
  padding: 24px;
  box-shadow: 0 20px 50px rgba(0, 0, 0, 0.45),
              inset 0 1px 0 rgba(255, 255, 255, 0.05);
}

/* ===== 显示区域 ===== */
.display {
  background: #1b1b2a;
  color: #f5f5f5;
  font-size: 2.4rem;
  font-weight: 300;
  text-align: right;
  padding: 20px 18px;
  border-radius: 12px;
  margin-bottom: 20px;
  min-height: 80px;
  word-wrap: break-word;
  word-break: break-all;
  overflow: hidden;
  box-shadow: inset 0 2px 8px rgba(0, 0, 0, 0.5);
  letter-spacing: 1px;
}

/* ===== 按键网格 ===== */
.keys {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 12px;
}

/* ===== 按键基础样式 ===== */
.key {
  border: none;
  outline: none;
  cursor: pointer;
  font-size: 1.4rem;
  font-weight: 500;
  color: #eaeaea;
  background: #3a3a52;
  border-radius: 12px;
  padding: 18px 0;
  transition: transform 0.08s ease, background 0.2s ease, box-shadow 0.2s ease;
  user-select: none;
  box-shadow: 0 4px 0 #26263a;
}

.key:hover {
  background: #45456a;
}

.key:active {
  transform: translateY(2px);
  box-shadow: 0 2px 0 #26263a;
}

/* ===== 运算符按钮 ===== */
.key-op {
  background: #4a4a78;
  color: #ffd166;
  font-weight: 600;
  box-shadow: 0 4px 0 #2e2e4d;
}

.key-op:hover {
  background: #5a5a92;
}

/* ===== 清除按钮 ===== */
.key-clear {
  background: #e63946;
  color: #fff;
  font-weight: 600;
  box-shadow: 0 4px 0 #a8232e;
}

.key-clear:hover {
  background: #ff4d5a;
}

/* ===== 等号按钮（跨两行） ===== */
.key-equals {
  grid-row: span 2;
  background: linear-gradient(180deg, #06d6a0, #04b184);
  color: #fff;
  font-weight: 700;
  font-size: 1.6rem;
  box-shadow: 0 4px 0 #028a66;
}

.key-equals:hover {
  background: linear-gradient(180deg, #0be0aa, #05c492);
}

/* ===== 0 按钮跨两列 ===== */
.key-zero {
  grid-column: span 2;
}

/* ===== 响应式 ===== */
@media (max-width: 400px) {
  .display {
    font-size: 1.9rem;
    padding: 16px 14px;
  }
  .key {
    font-size: 1.2rem;
    padding: 15px 0;
  }
}