Update message severity colors, fix navbar double border (#37019)

- Tweak serverity background and border colors
- Use default text color instead of per-severity text colors.
- Replace `saturate` filter with semibold font weight on message headers.
- Fix navbar double border when a notification is present.

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>

web_src/css/modules/navbar.css

@@ -7,6 +7,11 @@
   padding: 0 10px;
 }
 
+/* When notification message is present after navbar, hide border to avoid double border */
+#navbar:has(+ .ui.message) {
+  border-bottom: none;
+}
+
 #navbar .navbar-left,
 #navbar .navbar-right {
   display: flex;