fix(ui): prefer Compose env values and polish error pages

When a managed variable shares a Compose-defined key, show the
hardcoded Compose value as read-only and exclude it from managed
rows. Render Contact support as a button, use the collapsible
component for 419 proxy help, and style its list/code markers.

resources/css/app.css

@@ -1207,7 +1207,8 @@ html[data-theme="custom"] textarea:disabled {
     color: var(--color-accent);
 }
 
-.error-extra details ul {
+.error-extra details ul,
+.error-proxy-help ul {
     margin: 0.5rem 0 0;
     padding-left: 1.125rem;
     display: flex;
@@ -1215,7 +1216,8 @@ html[data-theme="custom"] textarea:disabled {
     gap: 0.375rem;
 }
 
-.error-extra details code {
+.error-extra details code,
+.error-proxy-help code {
     font-family: var(--font-mono);
     font-size: 0.75rem;
     padding: 0.05rem 0.3rem;