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/hooks/useDialog.ts

@@ -0,0 +1,85 @@
+/**
+ * @file useDialog.ts
+ * @description The one modal focus contract, shared by every dialog: focus
+ * moves in on open (the `initialFocus` element, else the panel's first
+ * focusable), Tab and Shift+Tab cycle inside the panel, Escape calls `onClose`,
+ * and focus returns to whatever held it before the dialog opened. Extracted
+ * from `ConfirmModal` so the other modals stop re-implementing (or skipping) it.
+ * Dialogs STACK: only the most recently opened one answers Escape and Tab, so
+ * a confirm opened over the sidebar drawer closes alone and traps focus alone.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { useEffect, useRef, type RefObject } from "react";
+
+/** Open dialogs, oldest first; only the last one handles keys. */
+const stack: object[] = [];
+
+const FOCUSABLE =
+  'button:not([disabled]), [href], input:not([disabled]), select:not([disabled]), textarea:not([disabled]), [tabindex]:not([tabindex="-1"])';
+
+/**
+ * Wire focus-in, a Tab trap, Escape and focus restore to a dialog panel.
+ * @param open Whether the dialog is shown.
+ * @param panelRef The dialog panel (the element carrying `role="dialog"`).
+ * @param onClose Called on Escape. Pass `undefined` for a dialog Escape must not close.
+ * @param initialFocus Element to focus on open; defaults to the first focusable.
+ */
+export function useDialog(
+  open: boolean,
+  panelRef: RefObject<HTMLElement>,
+  onClose?: () => void,
+  initialFocus?: RefObject<HTMLElement>
+): void {
+  // Read through a ref so an inline `onClose` does not re-run the effect, which
+  // would steal focus back to the first control on every parent render.
+  const closeRef = useRef(onClose);
+  closeRef.current = onClose;
+
+  useEffect(() => {
+    if (!open) return;
+    const token = {};
+    stack.push(token);
+    const previous = document.activeElement instanceof HTMLElement ? document.activeElement : null;
+    const timer = window.setTimeout(() => {
+      const panel = panelRef.current;
+      if (!panel || panel.contains(document.activeElement)) return;
+      (initialFocus?.current ?? panel.querySelector<HTMLElement>(FOCUSABLE) ?? panel).focus();
+    }, 0);
+
+    const onKey = (e: KeyboardEvent) => {
+      const panel = panelRef.current;
+      if (!panel || stack[stack.length - 1] !== token) return;
+      if (e.key === "Escape" && closeRef.current) {
+        e.preventDefault();
+        e.stopPropagation();
+        closeRef.current();
+        return;
+      }
+      if (e.key !== "Tab") return;
+      const items = panel.querySelectorAll<HTMLElement>(FOCUSABLE);
+      if (items.length === 0) return;
+      const first = items[0]!;
+      const last = items[items.length - 1]!;
+      if (
+        e.shiftKey &&
+        (document.activeElement === first || !panel.contains(document.activeElement))
+      ) {
+        e.preventDefault();
+        last.focus();
+      } else if (!e.shiftKey && document.activeElement === last) {
+        e.preventDefault();
+        first.focus();
+      }
+    };
+
+    document.addEventListener("keydown", onKey);
+    return () => {
+      window.clearTimeout(timer);
+      document.removeEventListener("keydown", onKey);
+      stack.splice(stack.indexOf(token), 1);
+      previous?.focus?.();
+    };
+    // eslint-disable-next-line react-hooks/exhaustive-deps -- refs are stable
+  }, [open]);
+}