feat(frontend): add shared banner system (error/warning/notice/success)

A plain module-level singleton store (banner.ts) plus a single renderer
(BannerStack, mounted once above LoginGate in main.tsx) -- no React
Context, since this codebase has none today and a plain exported
function is callable from anywhere, including api/client.ts (a plain
module, not a component) in a later task. Nothing calls it yet; that's
each of this plan's remaining tasks.

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

frontend/src/banner.ts

@@ -0,0 +1,54 @@
+export type BannerSeverity = "error" | "warning" | "notice" | "success";
+
+export interface BannerEntry {
+  id: number;
+  severity: BannerSeverity;
+  message: string;
+}
+
+const DISMISS_AFTER_MS = 8000;
+
+let banners: BannerEntry[] = [];
+let nextId = 1;
+const listeners = new Set<() => void>();
+
+function notify() {
+  for (const listener of listeners) listener();
+}
+
+function show(severity: BannerSeverity, message: string): void {
+  const id = nextId++;
+  banners = [{ id, severity, message }, ...banners];
+  notify();
+  setTimeout(() => dismiss(id), DISMISS_AFTER_MS);
+}
+
+export function showError(message: string): void {
+  show("error", message);
+}
+
+export function showWarning(message: string): void {
+  show("warning", message);
+}
+
+export function showNotice(message: string): void {
+  show("notice", message);
+}
+
+export function showSuccess(message: string): void {
+  show("success", message);
+}
+
+export function dismiss(id: number): void {
+  banners = banners.filter((b) => b.id !== id);
+  notify();
+}
+
+export function subscribe(listener: () => void): () => void {
+  listeners.add(listener);
+  return () => listeners.delete(listener);
+}
+
+export function getSnapshot(): BannerEntry[] {
+  return banners;
+}

frontend/src/components/BannerStack.css

@@ -0,0 +1,51 @@
+.banner-stack {
+  display: flex;
+  flex-direction: column;
+}
+
+.banner {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 1rem;
+  padding: 0.75rem 1.5rem;
+  white-space: pre-line;
+}
+
+.banner-message {
+  flex: 1;
+}
+
+.banner-close {
+  background: none;
+  border: none;
+  color: inherit;
+  font-size: 1.1rem;
+  line-height: 1;
+  cursor: pointer;
+  padding: 0 0.25rem;
+}
+
+.banner-error {
+  background: #7f1d1d;
+  border-bottom: 1px solid #f87171;
+  color: #fff;
+}
+
+.banner-warning {
+  background: #7c2d12;
+  border-bottom: 1px solid #fb923c;
+  color: #fff;
+}
+
+.banner-notice {
+  background: #1e3a5f;
+  border-bottom: 1px solid #3b82f6;
+  color: #fff;
+}
+
+.banner-success {
+  background: #14532d;
+  border-bottom: 1px solid #22c55e;
+  color: #fff;
+}

frontend/src/components/BannerStack.tsx

@@ -0,0 +1,26 @@
+import { useSyncExternalStore } from "react";
+import "./BannerStack.css";
+import { dismiss, getSnapshot, subscribe } from "../banner";
+
+// Mounted exactly once, in main.tsx, above <LoginGate /> -- see
+// docs/superpowers/specs/2026-07-27-modern-error-displays-design.md. Renders
+// in normal document flow (not position: fixed) so it pushes down whatever's
+// currently showing (login screen, onboarding wizard, or the full app),
+// without needing a separate mount point in each of those three layouts.
+export function BannerStack() {
+  const banners = useSyncExternalStore(subscribe, getSnapshot);
+  if (banners.length === 0) return null;
+
+  return (
+    <div className="banner-stack">
+      {banners.map((b) => (
+        <div key={b.id} className={`banner banner-${b.severity}`}>
+          <span className="banner-message">{b.message}</span>
+          <button type="button" className="banner-close" aria-label="Dismiss" onClick={() => dismiss(b.id)}>
+            ×
+          </button>
+        </div>
+      ))}
+    </div>
+  );
+}

frontend/src/main.tsx

@@ -1,9 +1,11 @@
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
+import { BannerStack } from './components/BannerStack.tsx'
 import { LoginGate } from './LoginGate.tsx'
 
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
+    <BannerStack />
     <LoginGate />
   </StrictMode>,
 )