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)

portal/app/templates/base.html

@@ -10,7 +10,7 @@
   {% if current_user.is_authenticated %}
   <header class="topbar">
     <div class="topbar-brand">
-      <span class="brand-icon">⬡</span>
+      <img src="{{ url_for('static', filename='Icon_platform.png') }}" alt="Platform Icon" class="brand-icon" />
       <span class="brand-name">Enterprise Digital Platform</span>
     </div>
     <nav class="topbar-nav">