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/explorer-sidebar.tsx

@@ -3,7 +3,6 @@ import {
   View,
   Text,
   Pressable,
-  Platform,
   useWindowDimensions,
   StyleSheet as RNStyleSheet,
 } from "react-native";
@@ -26,6 +25,7 @@ import { FileExplorerPane } from "./file-explorer-pane";
 import { useKeyboardShiftStyle } from "@/hooks/use-keyboard-shift-style";
 import { useWindowControlsPadding } from "@/utils/desktop-window";
 import { TitlebarDragRegion } from "@/components/desktop/titlebar-drag-region";
+import { isWeb } from "@/constants/platform";
 
 const MIN_CHAT_WIDTH = 400;
 function logExplorerSidebar(_event: string, _details: Record<string, unknown>): void {}
@@ -252,7 +252,7 @@ export function ExplorerSidebar({
 
   // Mobile: full-screen overlay with gesture.
   // On web, keep it interactive only while open so closed sidebars don't eat taps.
-  const overlayPointerEvents = Platform.OS === "web" ? (isOpen ? "auto" : "none") : "box-none";
+  const overlayPointerEvents = isWeb ? (isOpen ? "auto" : "none") : "box-none";
 
   // Navigation stacks can keep previous screens mounted; hide sidebars for unfocused
   // screens so only the active screen exposes explorer/terminal surfaces.
@@ -309,12 +309,7 @@ export function ExplorerSidebar({
       <View style={[styles.desktopSidebarBorder, { flex: 1 }]}>
         {/* Resize handle - absolutely positioned over left border */}
         <GestureDetector gesture={resizeGesture}>
-          <View
-            style={[
-              styles.resizeHandle,
-              Platform.OS === "web" && ({ cursor: "col-resize" } as any),
-            ]}
-          />
+          <View style={[styles.resizeHandle, isWeb && ({ cursor: "col-resize" } as any)]} />
         </GestureDetector>
 
         <SidebarContent