Add platform icons and fix dark mode contrast on HTTPS config page

- Portal: replace placeholder brand icon with Icon_platform.png in topbar
- NetworkView: replace placeholder sidebar logo with Icon_networkView.png
- DigiServer: fix https_config.html dark mode contrast (hardcoded light colors
  replaced with body.dark-mode CSS overrides and CSS variables)

NetworkView/frontend/src/components/Sidebar.tsx

@@ -102,7 +102,7 @@ export default function Sidebar() {
     <aside className="sidebar">
       <div className="sidebar-header">
         <Link to="/" className="sidebar-logo">
-          <span className="logo-icon">⬛</span>
+          <img src={`${import.meta.env.BASE_URL}Icon_networkView.png`} alt="NetworkView" className="logo-icon" />
           <span className="logo-text">NetworkView</span>
         </Link>
       </div>