Improve severity labels in Actions logs and tweak colors (#36993)

Add support for error, warning, notice, and debug log commands with bold
label prefixes and colored backgrounds matching GitHub's style. Parse
both `##[cmd]` and `::cmd args::` formats.

Also improved the severity colors globally and added a devtest page for
these.

---------

Co-authored-by: Claude (claude-opus-4-6) <noreply@anthropic.com>

web_src/css/base.css

@@ -393,12 +393,6 @@ img.ui.avatar,
   aspect-ratio: 1;
 }
 
-.ui.error.message .header,
-.ui.warning.message .header {
-  color: inherit;
-  filter: saturate(2);
-}
-
 .full.height {
   flex-grow: 1;
   padding-bottom: var(--page-space-bottom);