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/git-diff-pane.tsx

@@ -6,7 +6,6 @@ import {
   ActivityIndicator,
   Pressable,
   FlatList,
-  Platform,
   type LayoutChangeEvent,
   type NativeSyntheticEvent,
   type NativeScrollEvent,
@@ -79,6 +78,7 @@ import {
   formatDiffGutterText,
   hasVisibleDiffTokens,
 } from "@/utils/diff-rendering";
+import { isWeb, isNative } from "@/constants/platform";
 
 export type { GitActionId, GitAction, GitActions } from "@/components/git-actions-policy";
 
@@ -99,7 +99,7 @@ type WrappedWebTextStyle = TextStyle & {
 };
 
 function getWrappedTextStyle(wrapLines: boolean): WrappedWebTextStyle | undefined {
-  if (Platform.OS !== "web") {
+  if (isNative) {
     return undefined;
   }
   return wrapLines
@@ -453,7 +453,7 @@ const DiffFileHeader = memo(function DiffFileHeader({
         }}
         onPressOut={(event) => {
           if (
-            Platform.OS !== "web" &&
+            isNative &&
             !pressHandledRef.current &&
             layoutYRef.current === 0 &&
             pressInRef.current
@@ -632,8 +632,8 @@ export function GitDiffPane({ serverId, workspaceId, cwd, hideHeaderRow }: GitDi
   const { theme } = useUnistyles();
   const toast = useToast();
   const isMobile = useIsCompactFormFactor();
-  const showDesktopWebScrollbar = Platform.OS === "web" && !isMobile;
-  const canUseSplitLayout = Platform.OS === "web" && !isMobile;
+  const showDesktopWebScrollbar = isWeb && !isMobile;
+  const canUseSplitLayout = isWeb && !isMobile;
   const router = useRouter();
   const [diffModeOverride, setDiffModeOverride] = useState<"uncommitted" | "base" | null>(null);
   const [postShipArchiveSuggested, setPostShipArchiveSuggested] = useState(false);