/* --- CẤU HÌNH CHUNG --- */
    .fixed-contact-buttons {
        position: fixed;
        display: flex;
        flex-direction: column;
        z-index: 100;
    }

    .contact-btn {
        position: relative;
        border-radius: 50%;
        display: flex;
        align-items: center;
        justify-content: center;
        transition: all 0.3s ease;
        text-decoration: none;
        box-sizing: border-box; 
    }

    .contact-btn::before {
        content: "";
        position: absolute;
        top: 0; left: 0; right: 0; bottom: 0;
        border-radius: 50%;
        background: inherit;
        opacity: 0.4;
        z-index: -1;
        animation: pulse-wave 2s infinite;
    }

    @keyframes pulse-wave {
        0% { transform: scale(1); opacity: 0.4; }
        100% { transform: scale(1.8); opacity: 0; }
    }

    .contact-btn img {
        width: 100%; 
        height: auto;
        object-fit: contain;
        display: block;
    }

    /* --------------------------------------------------
       TẦNG 1: DESKTOP (> 1440px)
    -------------------------------------------------- */
    @media (min-width: 1440px) {
        .fixed-contact-buttons { right: 35px; bottom: 90px; gap: 15px; }
        .contact-btn { width: 45px; height: 45px; opacity: 0.8; }
        .zalo-btn { background: #1e73be; padding: 4px; } 
        .call-btn { background: #8B0000; padding: 8px; }
        .white-icon { filter: brightness(0) invert(1); }
    }

    /* --------------------------------------------------
       TẦNG 2: TABLET LỚN (820px - 1439px)
    -------------------------------------------------- */
    @media (max-width: 1439px) and (min-width: 820px) {
        .fixed-contact-buttons { right: 20px; bottom: 60px; gap: 15px; }
        .contact-btn { width: 50px; height: 50px; }
        .zalo-btn { background: #1e73be; padding: 10px; } 
        .call-btn { background: #8B0000; padding: 14px; }
        .white-icon { filter: brightness(0) invert(1); }
    }

    /* --------------------------------------------------
       TẦNG 3: TABLET NHỎ (768px - 819px) - GOLDEN PARAMETER
    -------------------------------------------------- */
    @media (max-width: 819px) and (min-width: 768px) {
        .fixed-contact-buttons { 
            right: 20px !important; 
            bottom: 40px !important;
            gap: 12px; 
        }
        .contact-btn { width: 52px; height: 52px; }
        .zalo-btn { background: #1e73be; padding: 10px; }
        .call-btn { background: #8B0000; padding: 14px; }
        .white-icon { filter: brightness(0) invert(1); }
    }

    /* --------------------------------------------------
       TẦNG 4: MOBILE (< 767px) - GOLDEN PARAMETER
    -------------------------------------------------- */
    @media (max-width: 767px) {
        .fixed-contact-buttons { 
            right: 20px !important; 
            bottom: 40px !important; 
            gap: 12px; 
        }
        .contact-btn { width: 40px; height: 40px; }
        .zalo-btn { background: #1e73be; padding: 4px; }
        .call-btn { 
            background: #8B0000; 
            padding: 8px; 
            animation: quick-shake 0.5s infinite; 
        }
        .white-icon { filter: brightness(0) invert(1); }
    }

    @keyframes quick-shake {
        0%, 100% { transform: rotate(0); }
        25% { transform: rotate(5deg); }
        75% { transform: rotate(-5deg); }
    }