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;
+}