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/__tests__/uiPrimitives.test.ts

@@ -0,0 +1,55 @@
+/**
+ * @file Tests for the shared UI primitives: the toast queue (errors stay,
+ * successes fade, duplicates collapse), promise confirms (a second ask cancels
+ * the first) and the IME-aware Enter check.
+ *
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { dismissToast, subscribeToasts, toast, type ToastItem } from "../toast";
+import { confirmDialog, settleConfirm, subscribeConfirm } from "../confirm";
+import { isSubmitEnter } from "../keys";
+
+afterEach(() => vi.useRealTimers());
+
+describe("toast", () => {
+  it("keeps an error, fades a success, collapses a repeat", () => {
+    vi.useFakeTimers();
+    let seen: ToastItem[] = [];
+    const off = subscribeToasts((i) => (seen = i));
+    const errId = toast.error(new Error("boom"));
+    toast.success("saved");
+    toast.success("saved");
+    expect(seen.map((t) => t.message)).toEqual(["boom", "saved"]);
+    vi.advanceTimersByTime(5000);
+    expect(seen.map((t) => t.message)).toEqual(["boom"]);
+    dismissToast(errId);
+    expect(seen).toEqual([]);
+    off();
+  });
+});
+
+describe("confirmDialog", () => {
+  it("resolves on settle and cancels an earlier pending ask", async () => {
+    const off = subscribeConfirm(() => {});
+    const first = confirmDialog({ title: "a" });
+    const second = confirmDialog({ title: "b" });
+    await expect(first).resolves.toBe(false);
+    settleConfirm(true);
+    await expect(second).resolves.toBe(true);
+    off();
+  });
+});
+
+describe("isSubmitEnter", () => {
+  const ev = (o: Partial<KeyboardEvent>) =>
+    ({ key: "Enter", shiftKey: false, isComposing: false, keyCode: 13, ...o }) as KeyboardEvent;
+  it("submits a plain Enter only", () => {
+    expect(isSubmitEnter(ev({}))).toBe(true);
+    expect(isSubmitEnter(ev({ shiftKey: true }))).toBe(false);
+    expect(isSubmitEnter(ev({ isComposing: true }))).toBe(false);
+    expect(isSubmitEnter(ev({ keyCode: 229 }))).toBe(false);
+    expect(isSubmitEnter(ev({ key: "a" }))).toBe(false);
+  });
+});