style: change header background from orange to dark cyan

Improves readability of the lightning bolt emoji and gives the header
a distinctive look.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/ui/widgets.py

@@ -26,7 +26,7 @@ class HeaderPanel(Static):
     HeaderPanel {
         dock: top;
         height: 1;
-        background: $accent;
+        background: darkcyan;
         color: $text;
         padding: 0 2;
     }