feat(frontend): error banners persist until manually dismissed

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

frontend/src/banner.ts

@@ -6,6 +6,8 @@ export interface BannerEntry {
   message: string;
 }
 
+// Non-error banners clean themselves up; errors stay until the user
+// dismisses them, so a failure can't silently vanish before it's read.
 const DISMISS_AFTER_MS = 8000;
 
 let banners: BannerEntry[] = [];
@@ -25,7 +27,9 @@ function show(severity: BannerSeverity, message: string): void {
   const id = nextId++;
   banners = [{ id, severity, message }, ...banners];
   notify();
-  setTimeout(() => dismiss(id), DISMISS_AFTER_MS);
+  if (severity !== "error") {
+    setTimeout(() => dismiss(id), DISMISS_AFTER_MS);
+  }
 }
 
 export function showError(message: string): void {