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/message.tsx

@@ -7,7 +7,6 @@ import {
   type LayoutChangeEvent,
   StyleProp,
   ViewStyle,
-  Platform,
 } from "react-native";
 import * as React from "react";
 import {
@@ -86,6 +85,7 @@ import { useToolCallSheet } from "./tool-call-sheet";
 import { ToolCallDetailsContent } from "./tool-call-details";
 import { useAttachmentPreviewUrl } from "@/attachments/use-attachment-preview-url";
 import type { DaemonClient } from "@server/client/daemon-client";
+import { isWeb, isNative } from "@/constants/platform";
 
 interface UserMessageProps {
   message: string;
@@ -134,7 +134,7 @@ const SCROLL_EDGE_EPSILON = 0.5;
 type ScrollAxis = "x" | "y";
 
 function ensureWebToolCallShimmerKeyframes() {
-  if (Platform.OS !== "web") {
+  if (isNative) {
     return;
   }
   if (typeof document === "undefined") {
@@ -341,12 +341,13 @@ export const UserMessage = memo(function UserMessage({
   isLastInGroup = true,
   disableOuterSpacing,
 }: UserMessageProps) {
+  const isCompact = useIsCompactFormFactor();
   const [messageHovered, setMessageHovered] = useState(false);
   const [copyButtonHovered, setCopyButtonHovered] = useState(false);
   const resolvedDisableOuterSpacing = useDisableOuterSpacing(disableOuterSpacing);
   const hasText = message.trim().length > 0;
   const hasImages = images.length > 0;
-  const showCopyButton = hasText && (Platform.OS !== "web" || messageHovered || copyButtonHovered);
+  const showCopyButton = hasText && (isCompact || messageHovered || copyButtonHovered);
 
   return (
     <View
@@ -361,8 +362,8 @@ export const UserMessage = memo(function UserMessage({
     >
       <Pressable
         style={userMessageStylesheet.content}
-        onHoverIn={Platform.OS === "web" ? () => setMessageHovered(true) : undefined}
-        onHoverOut={Platform.OS === "web" ? () => setMessageHovered(false) : undefined}
+        onHoverIn={() => setMessageHovered(true)}
+        onHoverOut={() => setMessageHovered(false)}
       >
         <View style={userMessageStylesheet.bubble}>
           {hasImages ? (
@@ -434,7 +435,7 @@ export const assistantMessageStylesheet = StyleSheet.create((theme) => ({
     color: theme.colors.foreground,
     fontFamily: Fonts.mono,
     fontSize: 13,
-    userSelect: Platform.OS === "web" ? "text" : "auto",
+    userSelect: isWeb ? "text" : "auto",
   },
   imageFrame: {
     width: "100%",
@@ -657,7 +658,7 @@ function MarkdownLink({
   children: ReactNode;
 }) {
   const [hovered, setHovered] = useState(false);
-  if (Platform.OS !== "web") {
+  if (isNative) {
     return (
       <Text accessibilityRole="link" onPress={() => onPress(href)} style={style}>
         {children}
@@ -779,8 +780,8 @@ export const TurnCopyButton = memo(function TurnCopyButton({
   return (
     <Pressable
       onPress={handleCopy}
-      onHoverIn={Platform.OS === "web" ? () => onHoverChange?.(true) : undefined}
-      onHoverOut={Platform.OS === "web" ? () => onHoverChange?.(false) : undefined}
+      onHoverIn={() => onHoverChange?.(true)}
+      onHoverOut={() => onHoverChange?.(false)}
       style={[turnCopyButtonStylesheet.container, containerStyle]}
       accessibilityRole="button"
       accessibilityLabel={
@@ -1060,7 +1061,7 @@ export const AssistantMessage = memo(function AssistantMessage({
             <Text
               key={node.key}
               onPress={() => parsed && onInlinePathPress?.(parsed)}
-              selectable={Platform.OS === "web" ? undefined : false}
+              selectable={isWeb ? undefined : false}
               style={[assistantMessageStylesheet.pathChip, assistantMessageStylesheet.pathChipText]}
             >
               {content}
@@ -1653,9 +1654,9 @@ const ExpandableBadge = memo(function ExpandableBadge({
     32,
     Math.min(120, labelRowWidth > 0 ? labelRowWidth * 0.28 : 0),
   );
-  const isWebShimmer = isLoading && Platform.OS === "web";
+  const isWebShimmer = isLoading && isWeb;
   const shouldMeasureWebShimmer = isWebShimmer;
-  const shouldMeasureNativeShimmer = isLoading && Platform.OS !== "web";
+  const shouldMeasureNativeShimmer = isLoading && isNative;
   const isNativeShimmer = shouldMeasureNativeShimmer && labelRowWidth > 0 && labelRowHeight > 0;
   const webShimmerSpanStartX = labelOffsetX;
   const webShimmerSpanEndX = secondaryLabel
@@ -1732,7 +1733,7 @@ const ExpandableBadge = memo(function ExpandableBadge({
   }, [isNativeShimmer, labelRowWidth, nativeShimmerPeakWidth, shimmerDuration, shimmerTranslateX]);
 
   useEffect(() => {
-    if (Platform.OS !== "web" || !isExpanded || !hasDetailContent) {
+    if (isNative || !isExpanded || !hasDetailContent) {
       return;
     }