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/left-sidebar.tsx

@@ -14,7 +14,6 @@ import {
   View,
   Pressable,
   Text,
-  Platform,
   useWindowDimensions,
   StyleSheet as RNStyleSheet,
 } from "react-native";
@@ -59,6 +58,7 @@ import {
   parseServerIdFromPathname,
 } from "@/utils/host-routes";
 import { useOpenProjectPicker } from "@/hooks/use-open-project-picker";
+import { isWeb } from "@/constants/platform";
 
 const MIN_CHAT_WIDTH = 400;
 
@@ -527,7 +527,7 @@ function MobileSidebar({
     pointerEvents: backdropOpacity.value > 0.01 ? "auto" : "none",
   }));
 
-  const overlayPointerEvents = Platform.OS === "web" ? (isOpen ? "auto" : "none") : "box-none";
+  const overlayPointerEvents = isWeb ? (isOpen ? "auto" : "none") : "box-none";
 
   return (
     <View style={StyleSheet.absoluteFillObject} pointerEvents={overlayPointerEvents}>
@@ -833,12 +833,7 @@ function DesktopSidebar({
 
         {/* Resize handle - absolutely positioned over right 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>
       </View>
     </Animated.View>