feat(ui): shared toast, confirm, dialog, copy and IME-safe Enter primitives

One implementation each, for every page to use instead of its own copy:

- lib/toast.ts + components/Toaster.tsx: toast.success|error|info from
  anywhere, rendered in polite/assertive live regions; an error stays until
  dismissed, a success fades, a repeat collapses.
- lib/confirm.ts + components/ConfirmHost.tsx: await confirmDialog({...})
  and confirmDiscard() through the themed ConfirmModal, replacing
  window.confirm and the actions that asked nothing.
- hooks/useDialog.ts: the modal focus contract (focus in, Tab trapped,
  Escape, focus restored), extracted from ConfirmModal; dialogs stack so
  only the top one answers keys. ConfirmModal now uses it.
- hooks/useArmed.ts: two-click confirmation that disarms after 3 s.
- hooks/useUnsavedGuard.ts: beforeunload while a draft is dirty.
- lib/clipboard.ts copyText: Clipboard API with an execCommand fallback for
  non-secure origins, outcome toasted.
- lib/keys.ts isSubmitEnter: ignores the Enter that confirms an IME
  candidate (Vietnamese Telex, CJK).

client/src/lib/toast.ts

@@ -0,0 +1,56 @@
+/**
+ * @file toast.ts
+ * @description App-wide toast queue: `toast.success(msg)` / `toast.error(msg)`
+ * from anywhere, rendered once by `components/Toaster.tsx` inside live regions.
+ * A success fades after a few seconds; an error stays until dismissed, because
+ * a message that vanishes before it is read is no message at all (WCAG 2.2.1).
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+export type ToastKind = "success" | "error" | "info";
+
+export interface ToastItem {
+  id: number;
+  kind: ToastKind;
+  message: string;
+}
+
+const SUCCESS_MS = 4000;
+let items: ToastItem[] = [];
+let nextId = 1;
+const listeners = new Set<(items: ToastItem[]) => void>();
+
+function emit() {
+  for (const l of listeners) l(items);
+}
+
+/** Remove one toast. */
+export function dismissToast(id: number): void {
+  items = items.filter((t) => t.id !== id);
+  emit();
+}
+
+function push(kind: ToastKind, message: string): number {
+  const id = nextId++;
+  // Same text twice in a row (a double click, a retry) shows once.
+  items = [...items.filter((t) => t.message !== message), { id, kind, message }].slice(-4);
+  emit();
+  if (kind !== "error") window.setTimeout(() => dismissToast(id), SUCCESS_MS);
+  return id;
+}
+
+/** Subscribe to the queue; returns the unsubscribe. */
+export function subscribeToasts(fn: (items: ToastItem[]) => void): () => void {
+  listeners.add(fn);
+  fn(items);
+  return () => {
+    listeners.delete(fn);
+  };
+}
+
+export const toast = {
+  success: (message: string) => push("success", message),
+  info: (message: string) => push("info", message),
+  error: (message: unknown) =>
+    push("error", message instanceof Error ? message.message : String(message)),
+};