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/welcome-screen.tsx

@@ -1,5 +1,5 @@
 import { useCallback, useEffect, useState, useSyncExternalStore } from "react";
-import { Pressable, Text, View, Platform, ScrollView } from "react-native";
+import { Pressable, Text, View, ScrollView } from "react-native";
 import { useRouter } from "expo-router";
 import { StyleSheet, useUnistyles } from "react-native-unistyles";
 import { QrCode, Link2, ClipboardPaste, ExternalLink } from "lucide-react-native";
@@ -18,6 +18,7 @@ import { formatVersionWithPrefix } from "@/desktop/updates/desktop-updates";
 import { buildHostRootRoute } from "@/utils/host-routes";
 import { PaseoLogo } from "@/components/icons/paseo-logo";
 import { openExternalUrl } from "@/utils/open-external-url";
+import { isWeb, isNative } from "@/constants/platform";
 
 type WelcomeAction = {
   key: "scan-qr" | "direct-connection" | "paste-pairing-link";
@@ -255,52 +256,51 @@ export function WelcomeScreen({ onHostAdded }: WelcomeScreenProps) {
     [router],
   );
 
-  const actions: WelcomeAction[] =
-    Platform.OS === "web"
-      ? [
-          {
-            key: "direct-connection",
-            label: "Direct connection",
-            testID: "welcome-direct-connection",
-            primary: true,
-            icon: Link2,
-            onPress: () => setIsDirectOpen(true),
-          },
-          {
-            key: "paste-pairing-link",
-            label: "Paste pairing link",
-            testID: "welcome-paste-pairing-link",
-            primary: false,
-            icon: ClipboardPaste,
-            onPress: () => setIsPasteLinkOpen(true),
-          },
-        ]
-      : [
-          {
-            key: "scan-qr",
-            label: "Scan QR code",
-            testID: "welcome-scan-qr",
-            primary: true,
-            icon: QrCode,
-            onPress: () => router.push("/pair-scan?source=onboarding"),
-          },
-          {
-            key: "direct-connection",
-            label: "Direct connection",
-            testID: "welcome-direct-connection",
-            primary: false,
-            icon: Link2,
-            onPress: () => setIsDirectOpen(true),
-          },
-          {
-            key: "paste-pairing-link",
-            label: "Paste pairing link",
-            testID: "welcome-paste-pairing-link",
-            primary: false,
-            icon: ClipboardPaste,
-            onPress: () => setIsPasteLinkOpen(true),
-          },
-        ];
+  const actions: WelcomeAction[] = isWeb
+    ? [
+        {
+          key: "direct-connection",
+          label: "Direct connection",
+          testID: "welcome-direct-connection",
+          primary: true,
+          icon: Link2,
+          onPress: () => setIsDirectOpen(true),
+        },
+        {
+          key: "paste-pairing-link",
+          label: "Paste pairing link",
+          testID: "welcome-paste-pairing-link",
+          primary: false,
+          icon: ClipboardPaste,
+          onPress: () => setIsPasteLinkOpen(true),
+        },
+      ]
+    : [
+        {
+          key: "scan-qr",
+          label: "Scan QR code",
+          testID: "welcome-scan-qr",
+          primary: true,
+          icon: QrCode,
+          onPress: () => router.push("/pair-scan?source=onboarding"),
+        },
+        {
+          key: "direct-connection",
+          label: "Direct connection",
+          testID: "welcome-direct-connection",
+          primary: false,
+          icon: Link2,
+          onPress: () => setIsDirectOpen(true),
+        },
+        {
+          key: "paste-pairing-link",
+          label: "Paste pairing link",
+          testID: "welcome-paste-pairing-link",
+          primary: false,
+          icon: ClipboardPaste,
+          onPress: () => setIsPasteLinkOpen(true),
+        },
+      ];
 
   const showHostList = hosts.length > 0 && !anyOnlineServerId;
 
@@ -321,7 +321,7 @@ export function WelcomeScreen({ onHostAdded }: WelcomeScreenProps) {
           {showHostList ? "Connecting to your hosts…" : "Connect to your host to start"}
         </Text>
 
-        {!showHostList && Platform.OS !== "web" && (
+        {!showHostList && isNative && (
           <>
             <Text style={styles.setupHint}>
               You need the Paseo desktop app or server running on your computer first.