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/desktop/permissions/desktop-permissions.ts

@@ -1,5 +1,5 @@
-import { Platform } from "react-native";
 import { getDesktopHost } from "@/desktop/host";
+import { isWeb, isNative } from "@/constants/platform";
 
 export type DesktopPermissionKind = "notifications" | "microphone";
 
@@ -45,7 +45,7 @@ type NavigatorLike = {
 };
 
 export function shouldShowDesktopPermissionSection(): boolean {
-  return Platform.OS === "web" && getDesktopHost() !== null;
+  return isWeb && getDesktopHost() !== null;
 }
 
 function status(input: DesktopPermissionStatus): DesktopPermissionStatus {
@@ -83,7 +83,7 @@ function isPermissionsQueryRuntimeUnsupported(error: unknown): boolean {
 }
 
 function getWebNotificationConstructor(): NotificationConstructorLike | null {
-  if (Platform.OS !== "web") {
+  if (isNative) {
     return null;
   }
   const NotificationConstructor = (globalThis as { Notification?: unknown }).Notification;
@@ -97,7 +97,7 @@ function getWebNotificationConstructor(): NotificationConstructorLike | null {
 }
 
 function getNavigatorLike(): NavigatorLike | null {
-  if (Platform.OS !== "web") {
+  if (isNative) {
     return null;
   }
   const webNavigator = (globalThis as { navigator?: unknown }).navigator;
@@ -133,7 +133,7 @@ function mapNotificationPermissionString(permission: string): DesktopPermissionS
 }
 
 async function getNotificationPermissionStatus(): Promise<DesktopPermissionStatus> {
-  if (Platform.OS !== "web") {
+  if (isNative) {
     return status({
       state: "unavailable",
       detail: "Desktop notification status is only available on web runtime.",
@@ -167,7 +167,7 @@ async function getNotificationPermissionStatus(): Promise<DesktopPermissionStatu
 }
 
 async function getMicrophonePermissionStatus(): Promise<DesktopPermissionStatus> {
-  if (Platform.OS !== "web") {
+  if (isNative) {
     return status({
       state: "unavailable",
       detail: "Desktop microphone status is only available on web runtime.",
@@ -237,7 +237,7 @@ async function getMicrophonePermissionStatus(): Promise<DesktopPermissionStatus>
 }
 
 async function requestNotificationPermissionStatus(): Promise<DesktopPermissionStatus> {
-  if (Platform.OS !== "web") {
+  if (isNative) {
     return status({
       state: "unavailable",
       detail: "Desktop notification requests are only available on web runtime.",
@@ -264,7 +264,7 @@ async function requestNotificationPermissionStatus(): Promise<DesktopPermissionS
 }
 
 async function requestMicrophonePermissionStatus(): Promise<DesktopPermissionStatus> {
-  if (Platform.OS !== "web") {
+  if (isNative) {
     return status({
       state: "unavailable",
       detail: "Desktop microphone requests are only available on web runtime.",

packages/app/src/desktop/updates/desktop-updates.ts

@@ -1,6 +1,6 @@
-import { Platform } from "react-native";
 import { isElectronRuntime } from "@/desktop/host";
 import { invokeDesktopCommand } from "@/desktop/electron/invoke";
+import { isWeb } from "@/constants/platform";
 
 export interface DesktopAppUpdateCheckResult {
   hasUpdate: boolean;
@@ -50,7 +50,7 @@ function toNumberOr(defaultValue: number, value: unknown): number {
 }
 
 export function shouldShowDesktopUpdateSection(): boolean {
-  return Platform.OS === "web" && isElectronRuntime();
+  return isWeb && isElectronRuntime();
 }
 
 export function parseLocalDaemonVersionResult(raw: unknown): LocalDaemonVersionResult {