fix: Explicitly handle both http:// and https:// external links (qa-requested)

Fixes:
- Updated CSS selectors to explicitly target both http:// and https:// protocols
- Changed from a[href^="http"] to a[href^="http://"], a[href^="https://"]
- Makes code more self-documenting and explicit about handling both protocols

Verified:
- CSS syntax valid
- Selector matches both http:// and https:// links in resources.md
- Both light and dark mode selectors updated

QA Fix Session: 0

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Stephan Wald

src/css/custom.css

@@ -30,7 +30,8 @@
 }
 
 /* External link icon - targets links starting with http:// or https:// */
-a[href^="http"]::after {
+a[href^="http://"]::after,
+a[href^="https://"]::after {
   content: "";
   display: inline-block;
   width: 0.875em;
@@ -43,6 +44,7 @@ a[href^="http"]::after {
 }
 
 /* Dark mode external link icon - lighter color for better contrast */
-[data-theme='dark'] a[href^="http"]::after {
+[data-theme='dark'] a[href^="http://"]::after,
+[data-theme='dark'] a[href^="https://"]::after {
   background-image: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" fill="%23b4b4b4"><path d="M14 2.5a.5.5 0 0 0-.5-.5h-6a.5.5 0 0 0 0 1h4.793L2.146 13.146a.5.5 0 0 0 .708.708L13 3.707V8.5a.5.5 0 0 0 1 0z"/></svg>');
 }