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/constants/layout.ts

@@ -1,6 +1,5 @@
-import { Platform } from "react-native";
 import { useUnistyles } from "react-native-unistyles";
-import { isElectronRuntime, isElectronRuntimeMac } from "@/desktop/host";
+import { isWeb } from "@/constants/platform";
 
 export const FOOTER_HEIGHT = 75;
 
@@ -24,43 +23,10 @@ export const DESKTOP_TRAFFIC_LIGHT_HEIGHT = 45;
 export const DESKTOP_WINDOW_CONTROLS_WIDTH = 140;
 export const DESKTOP_WINDOW_CONTROLS_HEIGHT = 48;
 
-// Check if running in the Electron desktop runtime (any OS)
-function isElectronDesktopRuntime(): boolean {
-  if (Platform.OS !== "web") return false;
-  return isElectronRuntime();
-}
-
-// Check if running in the Electron desktop runtime on macOS
-function isElectronDesktopRuntimeMac(): boolean {
-  if (Platform.OS !== "web") return false;
-  return isElectronRuntimeMac();
-}
-
-// Cached result - only cache true, keep checking if false (in case desktop globals load later)
-let _isElectronRuntimeMacCached: boolean | null = null;
-let _isElectronRuntimeCached: boolean | null = null;
-
-export function getIsElectronRuntimeMac(): boolean {
-  if (_isElectronRuntimeMacCached === true) {
-    return true;
-  }
-  const result = isElectronDesktopRuntimeMac();
-  if (result) {
-    _isElectronRuntimeMacCached = true;
-  }
-  return result;
-}
-
-export function getIsElectronRuntime(): boolean {
-  if (_isElectronRuntimeCached === true) {
-    return true;
-  }
-  const result = isElectronDesktopRuntime();
-  if (result) {
-    _isElectronRuntimeCached = true;
-  }
-  return result;
-}
+export {
+  getIsElectron as getIsElectronRuntime,
+  getIsElectronMac as getIsElectronRuntimeMac,
+} from "./platform";
 
 /**
  * Reactive hook — re-renders the component when the breakpoint changes.
@@ -75,5 +41,5 @@ export function useIsCompactFormFactor(): boolean {
 // Keep that capability distinct from desktop-width layout so touch tablets
 // can use the desktop shell without entering web-only code paths.
 export function supportsDesktopPaneSplits(): boolean {
-  return Platform.OS === "web";
+  return isWeb;
 }