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/hooks/use-client-activity.ts

@@ -1,6 +1,7 @@
 import { useEffect, useRef, useCallback } from "react";
-import { AppState, Platform } from "react-native";
+import { AppState } from "react-native";
 import type { DaemonClient } from "@server/client/daemon-client";
+import { isWeb, isNative } from "@/constants/platform";
 
 const HEARTBEAT_INTERVAL_MS = 15_000;
 const ACTIVITY_HEARTBEAT_THROTTLE_MS = 5_000;
@@ -32,7 +33,7 @@ export function useClientActivity({
   const prevFocusedAgentIdRef = useRef<string | null>(focusedAgentId);
   const lastImmediateHeartbeatAtRef = useRef<number>(0);
 
-  const deviceType = Platform.OS === "web" ? "web" : "mobile";
+  const deviceType = isWeb ? "web" : "mobile";
 
   const recordUserActivity = useCallback(() => {
     lastActivityAtRef.current = new Date();
@@ -96,7 +97,7 @@ export function useClientActivity({
 
   // Track user activity on web for accurate staleness.
   useEffect(() => {
-    if (Platform.OS !== "web") return;
+    if (isNative) return;
     if (typeof document === "undefined") return;
 
     const handleUserActivity = () => {