refactor(ui): name the global overlay z-index layers (native-cinny nit)

Centralized the global floating-UI stacking values into styles/zIndex.ts
(inCallBanner 9990 < seasonalEffect 9997 < nightLight 9998 < toast 10001;
folds modals sit at 9999 between). Same values, no behavior change — just
removes the magic numbers and documents the layering so future overlays don't
collide. Component-internal small z-index stays local.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/toast/LotusToastContainer.tsx

@@ -4,6 +4,7 @@ import { color, config, Icon, IconButton, Icons } from 'folds';
 import { toastQueueAtom, dismissToastAtom, ToastNotif } from '../../state/toast';
 import { useSetting } from '../../state/hooks/settings';
 import { settingsAtom } from '../../state/settings';
+import { zIndices } from '../../styles/zIndex';
 
 // Inject the keyframe animation once
 const STYLE_ID = 'lotus-toast-keyframes';
@@ -214,7 +215,7 @@ export function LotusToastContainer() {
     position: 'fixed',
     bottom: '1.5rem',
     right: '1.5rem',
-    zIndex: 10001,
+    zIndex: zIndices.toast,
     display: 'flex',
     flexDirection: 'column',
     gap: config.space.S200,