Show a green dot for online screens in the screen list (0.2.30)

The status line under each screen name used the muted text colour for the
online dot, so online and offline screens looked alike. The dot is now its
own span and turns green when the screen is online; offline keeps the grey
open circle. Firmware 0.2.25 for both boards carries only the new version.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

screen_manager/app/static/style.css

@@ -185,6 +185,9 @@ aside h2 {
 .screen-main small {
   margin-top: 3px;
 }
+.screen-main .dot.online {
+  color: #2e9d62;
+}
 .screen-update {
   display: flex;
   align-items: center;