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/styles/zIndex.ts

@@ -0,0 +1,16 @@
+/**
+ * Global overlay stacking layers, centralized so floating Lotus UI doesn't
+ * collide. (folds `Overlay`/`Dialog` modals resolve to 9999, which sits between
+ * `nightLight` and `toast`.) Component-internal stacking uses small local
+ * z-index values and is intentionally not listed here.
+ */
+export const zIndices = {
+  /** In-call incoming-call banner — below seasonal/night-light/modals. */
+  inCallBanner: 9990,
+  /** Seasonal particle effect — below the night-light tint so particles tint. */
+  seasonalEffect: 9997,
+  /** Night Light tint overlay — above effects, below modals. */
+  nightLight: 9998,
+  /** Toasts — above everything, including modals. */
+  toast: 10001,
+} as const;