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-favicon-status.ts

@@ -1,5 +1,4 @@
 import { useEffect, useRef, useState } from "react";
-import { Platform } from "react-native";
 import { useShallow } from "zustand/shallow";
 import { getIsElectronRuntimeMac } from "@/constants/layout";
 import { useAggregatedAgents } from "./use-aggregated-agents";
@@ -9,6 +8,7 @@ import {
   deriveMacDockBadgeCountFromWorkspaceStatuses,
   type DesktopBadgeWorkspaceStatus,
 } from "@/utils/desktop-badge-state";
+import { isNative } from "@/constants/platform";
 
 type FaviconStatus = "none" | "running" | "attention";
 type ColorScheme = "dark" | "light";
@@ -76,18 +76,14 @@ function updateFavicon(status: FaviconStatus, colorScheme: ColorScheme) {
 }
 
 function getSystemColorScheme(): ColorScheme {
-  if (
-    Platform.OS !== "web" ||
-    typeof window === "undefined" ||
-    typeof window.matchMedia !== "function"
-  ) {
+  if (isNative || typeof window === "undefined" || typeof window.matchMedia !== "function") {
     return "dark";
   }
   return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
 }
 
 async function updateMacDockBadge(count?: number) {
-  if (Platform.OS !== "web" || !getIsElectronRuntimeMac()) return;
+  if (isNative || !getIsElectronRuntimeMac()) return;
 
   const desktopWindow = getDesktopHost()?.window?.getCurrentWindow?.();
   if (!desktopWindow || typeof desktopWindow.setBadgeCount !== "function") {
@@ -119,7 +115,7 @@ export function useFaviconStatus() {
 
   // Listen for system color scheme changes
   useEffect(() => {
-    if (Platform.OS !== "web" || typeof window === "undefined") return;
+    if (isNative || typeof window === "undefined") return;
 
     const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
     const handler = (e: MediaQueryListEvent) => {
@@ -132,7 +128,7 @@ export function useFaviconStatus() {
 
   // Update favicon when agents or color scheme changes
   useEffect(() => {
-    if (Platform.OS !== "web") return;
+    if (isNative) return;
 
     const status = deriveFaviconStatus(agents);
     updateFavicon(status, colorScheme);