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/stores/panel-store.ts

@@ -1,7 +1,6 @@
 import { create } from "zustand";
 import { persist, createJSONStorage } from "zustand/middleware";
 import AsyncStorage from "@react-native-async-storage/async-storage";
-import { Platform } from "react-native";
 import {
   buildExplorerCheckoutKey,
   coerceExplorerTabForCheckout,
@@ -9,6 +8,7 @@ import {
   resolveExplorerTabForCheckout,
   type ExplorerTab,
 } from "./explorer-tab-memory";
+import { isWeb } from "@/constants/platform";
 export type { ExplorerTab } from "./explorer-tab-memory";
 
 /**
@@ -128,7 +128,7 @@ function resolveExplorerTabFromActiveCheckout(state: PanelState): ExplorerTab |
   });
 }
 
-const DEFAULT_DESKTOP_OPEN = Platform.OS === "web";
+const DEFAULT_DESKTOP_OPEN = isWeb;
 
 export const usePanelStore = create<PanelState>()(
   persist(
@@ -318,11 +318,7 @@ export const usePanelStore = create<PanelState>()(
         const state = persistedState as Partial<PanelState> & Record<string, unknown>;
 
         if (version < 2) {
-          if (
-            Platform.OS === "web" &&
-            typeof state.explorerWidth === "number" &&
-            state.explorerWidth === 400
-          ) {
+          if (isWeb && typeof state.explorerWidth === "number" && state.explorerWidth === 400) {
             state.explorerWidth = DEFAULT_EXPLORER_SIDEBAR_WIDTH;
           }
 
@@ -337,7 +333,7 @@ export const usePanelStore = create<PanelState>()(
 
         if (version < 3) {
           if (
-            Platform.OS === "web" &&
+            isWeb &&
             typeof state.explorerWidth === "number" &&
             (state.explorerWidth === 400 || state.explorerWidth === 520)
           ) {