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/pages/App.tsx

@@ -27,6 +27,7 @@ import { LotusToastContainer } from '../features/toast/LotusToastContainer';
 import { useTauriNotificationBadge } from '../hooks/useTauriNotificationBadge';
 import { SeasonalEffect } from '../components/seasonal/SeasonalEffect';
 import { applyCustomAccent, removeCustomAccent } from '../utils/accentColor';
+import { zIndices } from '../styles/zIndex';
 
 const FONT_MAP: Record<string, string> = {
   system: "system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif",
@@ -95,7 +96,7 @@ function NightLightOverlay() {
         position: 'fixed',
         inset: 0,
         pointerEvents: 'none',
-        zIndex: 9998,
+        zIndex: zIndices.nightLight,
         backgroundColor: `rgba(255, 140, 0, ${(settings.nightLightOpacity ?? 30) / 100})`,
       }}
     />