Centralize platform gating and fix iPad/tablet support

Add `packages/app/src/constants/platform.ts` with canonical gates
(isWeb, isNative, getIsElectron) and refactor all ~100 ad-hoc
Platform.OS checks across 69 files to use shared imports.

Fix sidebar hover crash on native (onPointerEnter → onHoverIn),
fix tooltip to use breakpoint instead of platform detection, make
sidebar action buttons always visible on native where hover doesn't
work, and document the platform gating decision matrix in CLAUDE.md.

packages/app/src/components/use-web-scrollbar.tsx

@@ -1,6 +1,5 @@
 import { useCallback, useEffect, useState, type ReactNode, type RefObject } from "react";
 import {
-  Platform,
   type FlatList,
   type LayoutChangeEvent,
   type NativeScrollEvent,
@@ -12,6 +11,7 @@ import {
   useWebDesktopScrollbarMetrics,
   type ScrollbarMetrics,
 } from "./web-desktop-scrollbar";
+import { isWeb as platformIsWeb } from "@/constants/platform";
 
 const METRICS_EPSILON = 0.5;
 const HIDE_SCROLLBAR_STYLE_ID = "paseo-hide-scrollbar";
@@ -45,8 +45,7 @@ export function useWebElementScrollbar(
     contentRef?: RefObject<HTMLElement | null>;
   },
 ): ReactNode {
-  const isWeb = Platform.OS === "web";
-  const enabled = (options?.enabled ?? true) && isWeb;
+  const enabled = (options?.enabled ?? true) && platformIsWeb;
   const contentRef = options?.contentRef;
 
   const [metrics, setMetrics] = useState<ScrollbarMetrics>({
@@ -125,8 +124,7 @@ export function useWebScrollViewScrollbar(
   scrollableRef: RefObject<ScrollView | FlatList | null>,
   options?: { enabled?: boolean },
 ): WebScrollViewScrollbar {
-  const isWeb = Platform.OS === "web";
-  const enabled = (options?.enabled ?? true) && isWeb;
+  const enabled = (options?.enabled ?? true) && platformIsWeb;
   const metricsHook = useWebDesktopScrollbarMetrics();
 
   const onScrollToOffset = useCallback(