* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

body {
    font-family: -apple-system, "Segoe UI", "Microsoft YaHei", sans-serif;
    background: #f5f7fa;
    color: #333;
    min-height: 100vh;
    display: flex;
    justify-content: center;
    padding: 30px 15px;
}

.container {
    width: 100%;
    max-width: 520px;
}

h1 {
    font-size: 22px;
    text-align: center;
    margin-bottom: 6px;
    color: #2c3e50;
}

.subtitle {
    font-size: 13px;
    text-align: center;
    color: #888;
    margin-bottom: 20px;
}

/* ============ 计算器 ============ */

.calculator {
    background: #ffffff;
    border-radius: 14px;
    box-shadow: 0 4px 16px rgba(0, 0, 0, 0.06);
    padding: 20px;
    margin-bottom: 24px;
}

.display {
    background: #f0f3f8;
    border-radius: 10px;
    padding: 14px 16px;
    margin-bottom: 16px;
    min-height: 100px;
}

#expression {
    width: 100%;
    border: none;
    background: transparent;
    font-size: 22px;
    color: #222;
    outline: none;
    font-family: "Consolas", monospace;
}

#expression::placeholder {
    color: #aab;
    font-size: 16px;
}

.result {
    font-size: 26px;
    color: #2c3e50;
    text-align: right;
    min-height: 32px;
    margin-top: 8px;
    word-break: break-all;
    font-weight: 600;
}

.result.error {
    color: #e74c3c;
    font-size: 16px;
    font-weight: 400;
}

.message {
    font-size: 13px;
    color: #888;
    text-align: right;
    min-height: 18px;
    margin-top: 4px;
}

/* ============ 按钮 ============ */

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
}

.btn {
    padding: 16px 0;
    font-size: 20px;
    border: none;
    border-radius: 8px;
    background: #eef1f6;
    color: #333;
    cursor: pointer;
    transition: background 0.15s, transform 0.05s;
    font-family: inherit;
}

.btn:hover {
    background: #e0e5ee;
}

.btn:active {
    transform: scale(0.97);
}

.btn.op {
    background: #d8e4f5;
    color: #2c5282;
    font-weight: 600;
}

.btn.op:hover {
    background: #c5d7ee;
}

.btn.equal {
    background: #4a90e2;
    color: white;
    font-weight: 600;
    grid-column: span 4;
}

.btn.equal:hover {
    background: #3a7bc8;
}

/* ============ 历史记录 ============ */

.history-section {
    background: #ffffff;
    border-radius: 14px;
    box-shadow: 0 4px 16px rgba(0, 0, 0, 0.06);
    padding: 20px;
}

.history-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 14px;
}

.history-header h2 {
    font-size: 17px;
    color: #2c3e50;
}

.btn-clear {
    background: #fcebeb;
    color: #c0392b;
    border: none;
    padding: 7px 14px;
    border-radius: 6px;
    cursor: pointer;
    font-size: 13px;
    font-family: inherit;
    transition: background 0.15s;
}

.btn-clear:hover {
    background: #f8d7d7;
}

.history-loading,
.history-empty {
    text-align: center;
    color: #999;
    font-size: 14px;
    padding: 20px 0;
}

.history-list {
    list-style: none;
}

.history-item {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 12px 14px;
    border-radius: 8px;
    margin-bottom: 8px;
    background: #f7f9fc;
    transition: background 0.15s;
}

.history-item:hover {
    background: #eef3fa;
}

.history-info {
    flex: 1;
    min-width: 0;
}

.history-expr {
    font-family: "Consolas", monospace;
    font-size: 15px;
    color: #2c3e50;
    word-break: break-all;
}

.history-result {
    font-size: 14px;
    color: #4a90e2;
    margin-top: 3px;
    font-weight: 600;
}

.history-time {
    font-size: 12px;
    color: #999;
    margin-top: 3px;
}

.btn-delete {
    background: transparent;
    border: none;
    color: #c0392b;
    cursor: pointer;
    font-size: 16px;
    padding: 6px 10px;
    border-radius: 6px;
    margin-left: 10px;
    transition: background 0.15s;
}

.btn-delete:hover {
    background: #fcebeb;
}