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/keys.ts

@@ -0,0 +1,15 @@
+/**
+ * @file keys.ts
+ * @description Keyboard helpers shared by every single-line submit field.
+ * `isSubmitEnter` ignores the Enter that confirms an IME candidate (Vietnamese
+ * Telex, CJK): without it a rename or commit message is committed half-typed.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import type { KeyboardEvent as ReactKeyboardEvent } from "react";
+
+/** True for a plain Enter that should submit — not Shift+Enter, not mid-IME. */
+export function isSubmitEnter(e: ReactKeyboardEvent | KeyboardEvent): boolean {
+  const native = "nativeEvent" in e ? e.nativeEvent : e;
+  return e.key === "Enter" && !e.shiftKey && !native.isComposing && native.keyCode !== 229;
+}