dashboard: use casesladder scan_bg.gif scanline texture for header

Replace the generated grey tile with the scan_bg.gif background from
https://www.casesladder.com/ (https://img.igl.net/home/scan_bg.gif).

Author: Jeremy Ewing

api/static/images/header-bg.gif

@@ -42,7 +42,7 @@
   --btn3-background-color: #172033;
 }
 body { font-family: Exo, system-ui, sans-serif; background: var(--background-color); color: var(--text-color); }
-.header { background-color: var(--header-bg-color); background-image: url("/images/header-bg.png"); background-repeat: repeat; background-size: 4px 4px; border-bottom: 1px solid var(--border-color); }
+.header { background-color: var(--header-bg-color); background-image: url("/images/header-bg.gif"); background-repeat: repeat; border-bottom: 1px solid var(--border-color); }
 .header .container { padding: 20px 32px 0; max-width: 1600px; margin: 0 auto; }
 .header .container .logo { background-image: url("/images/logo-luxmon.svg") !important; width: 160px; height: 40px; background-repeat: no-repeat; background-size: contain; margin-bottom: 20px; }
 .header .container .menu .menu-item { background-color: var(--header-tab-unselected); color: var(--text-color); border-radius: 6px 6px 0 0; }