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-input.tsx

@@ -9,7 +9,6 @@ import {
   TextInputKeyPressEventData,
   TextInputSelectionChangeEventData,
   Image,
-  Platform,
   BackHandler,
 } from "react-native";
 import {
@@ -48,6 +47,7 @@ import {
   markScrollInvestigationEvent,
   markScrollInvestigationRender,
 } from "@/utils/scroll-jank-investigation";
+import { isWeb } from "@/constants/platform";
 
 export type ImageAttachment = AttachmentMetadata;
 
@@ -115,7 +115,7 @@ export interface MessageInputRef {
 
 const MIN_INPUT_HEIGHT = 30;
 const MAX_INPUT_HEIGHT = 160;
-const IS_WEB = Platform.OS === "web";
+const IS_WEB = isWeb;
 
 type WebTextInputKeyPressEvent = NativeSyntheticEvent<
   TextInputKeyPressEventData & {
@@ -1290,7 +1290,7 @@ const styles = StyleSheet.create(((theme: any) => ({
   rightButtonGroup: {
     flexDirection: "row",
     alignItems: "center",
-    gap: Platform.OS === "web" ? theme.spacing[2] : theme.spacing[1],
+    gap: isWeb ? theme.spacing[2] : theme.spacing[1],
   },
   attachButton: {
     width: 28,