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/components/seasonal/SeasonalEffect.tsx

@@ -1,6 +1,7 @@
 import React, { useMemo } from 'react';
 import { useAtomValue } from 'jotai';
 import { settingsAtom } from '../../state/settings';
+import { zIndices } from '../../styles/zIndex';
 import {
   animSeasonFall,
   animLeafFall,
@@ -758,7 +759,7 @@ function SeasonalOverlay({ theme, reduced }: { theme: SeasonTheme; reduced: bool
         pointerEvents: 'none',
         // Below the Night Light overlay (9998) so seasonal particles are tinted
         // by it, and below modals (9999) so dialogs are never obscured.
-        zIndex: 9997,
+        zIndex: zIndices.seasonalEffect,
         overflow: 'hidden',
       }}
     >