    /* Shared app navigation header (see _app_nav_header.html) */
    .header {
      display: flex;
      justify-content: space-between;
      align-items: center;
      border-bottom: 2px solid black;
      padding-bottom: 15px;
      margin-bottom: 20px;
    }
    .header-left {
      display: flex;
      flex-direction: column;
    }
    .header h1 {
      font-size: 24px;
      font-weight: bold;
    }
    .hdr-action-btn {
      display: inline-block;
      padding: 6px 12px;
      border: 2px solid #000;
      background: #fff;
      color: #000;
      text-decoration: none;
      font-size: 12px;
      font-weight: bold;
      font-family: "Courier New", monospace;
      text-transform: uppercase;
      box-shadow: 0 2px 8px rgba(0, 0, 0, 0.08), 0 1px 2px rgba(0, 0, 0, 0.06);
      transition:
        transform 0.15s ease-out,
        box-shadow 0.15s ease-out,
        background 0.15s ease-out;
    }
    .hdr-action-btn:hover {
      background: #ebebeb;
      box-shadow: 0 4px 14px rgba(0, 0, 0, 0.1), 0 2px 4px rgba(0, 0, 0, 0.06);
      text-decoration: none;
      color: #000;
    }
    .hdr-action-btn:active {
      transform: translateY(1px);
      background: #e2e2e2;
      box-shadow: 0 1px 4px rgba(0, 0, 0, 0.08);
      transition-duration: 0.08s;
    }
    .hdr-action-btn:focus-visible {
      outline: 2px solid #000;
      outline-offset: 3px;
    }
    [data-theme="dark"] .header { border-color: #444; }
    [data-theme="dark"] .hdr-action-btn {
      background: #1e1e2e;
      color: #e0e0e0;
      border-color: #666;
      box-shadow: 0 2px 10px rgba(0, 0, 0, 0.4), 0 1px 3px rgba(0, 0, 0, 0.35);
    }
    [data-theme="dark"] .hdr-action-btn:hover {
      background: #2c2c3c;
      border-color: #888;
      color: #e0e0e0;
      box-shadow: 0 4px 16px rgba(0, 0, 0, 0.5), 0 2px 6px rgba(0, 0, 0, 0.35);
    }
    [data-theme="dark"] .hdr-action-btn:active {
      background: #333;
      box-shadow: 0 1px 5px rgba(0, 0, 0, 0.45);
    }
    [data-theme="dark"] .hdr-action-btn:focus-visible {
      outline-color: #e0e0e0;
    }
