[FIR] Improve constraint errors rendering in html dump

compiler/fir/dump/resources/org/jetbrains/kotlin/fir/dump/colors.dark.css

@@ -43,6 +43,10 @@ body.dark {
     text-decoration-color: red;
 }
 
+.dark .subtype-error {
+    color: red;
+}
+
 .dark .error.unsupported {
     text-decoration-color: cyan;
 }

compiler/fir/dump/resources/org/jetbrains/kotlin/fir/dump/colors.white.css

@@ -37,6 +37,10 @@
     text-decoration-color: red;
 }
 
+.white .subtype-error {
+    color: red;
+}
+
 .white .error.unsupported {
     text-decoration-color: cyan;
 }

compiler/fir/dump/src/org/jetbrains/kotlin/fir/dump/HtmlFirDump.kt

@@ -1221,11 +1221,12 @@ class HtmlFirDump internal constructor(private var linkResolver: FirLinkResolver
                         when (callDiagnostic) {
                             is NewConstraintError -> {
                                 ident()
-                                +"Lower: "
+
                                 generate(callDiagnostic.lowerType as ConeKotlinType)
-                                br
-                                ident()
-                                +"Upper: "
+
+                                ws
+                                span(classes = "subtype-error") { +"<:" }
+                                ws
                                 generate(callDiagnostic.upperType as ConeKotlinType)
                             }
                             else -> {