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/web-desktop-scrollbar.tsx

@@ -1,7 +1,6 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from "react";
 import {
   PanResponder,
-  Platform,
   View,
   type LayoutChangeEvent,
   type NativeScrollEvent,
@@ -12,6 +11,7 @@ import {
   computeScrollOffsetFromDragDelta,
   computeVerticalScrollbarGeometry,
 } from "./web-desktop-scrollbar.math";
+import { isWeb as platformIsWeb } from "@/constants/platform";
 
 const METRICS_EPSILON = 0.5;
 const HANDLE_WIDTH_IDLE = 6;
@@ -135,7 +135,6 @@ export function WebDesktopScrollbarOverlay({
     maxScrollOffset: 0,
   });
   const onScrollToOffsetRef = useRef(onScrollToOffset);
-  const isWeb = Platform.OS === "web";
 
   const maxScrollOffset = Math.max(0, metrics.contentSize - metrics.viewportSize);
   const normalizedOffset = inverted
@@ -258,7 +257,7 @@ export function WebDesktopScrollbarOverlay({
   );
 
   const panResponder = useMemo(() => {
-    if (isWeb) {
+    if (platformIsWeb) {
       return null;
     }
 
@@ -280,11 +279,11 @@ export function WebDesktopScrollbarOverlay({
         setIsDragging(false);
       },
     });
-  }, [applyDragDelta, isWeb]);
+  }, [applyDragDelta, platformIsWeb]);
 
   const startWebDrag = useCallback(
     (event: any) => {
-      if (!isWeb) {
+      if (!platformIsWeb) {
         return;
       }
       const clientY = readClientY(event);
@@ -298,7 +297,7 @@ export function WebDesktopScrollbarOverlay({
       dragStartClientYRef.current = clientY;
       setIsDragging(true);
     },
-    [isWeb],
+    [platformIsWeb],
   );
 
   const handleGrabHoverIn = useCallback(() => {
@@ -313,7 +312,7 @@ export function WebDesktopScrollbarOverlay({
   }, []);
 
   useEffect(() => {
-    if (!isWeb || !isDragging) {
+    if (!platformIsWeb || !isDragging) {
       return;
     }
 
@@ -335,7 +334,7 @@ export function WebDesktopScrollbarOverlay({
       window.removeEventListener("pointerup", stopDragging);
       window.removeEventListener("pointercancel", stopDragging);
     };
-  }, [applyDragDelta, isDragging, isWeb]);
+  }, [applyDragDelta, isDragging, platformIsWeb]);
 
   if (!enabled || !geometry.isVisible) {
     return null;
@@ -371,7 +370,7 @@ export function WebDesktopScrollbarOverlay({
             height: thumbRegionHeight,
             transform: [{ translateY: thumbRegionOffset }],
           },
-          isWeb &&
+          platformIsWeb &&
             ({
               cursor: handleCursor,
               touchAction: "none",
@@ -383,7 +382,7 @@ export function WebDesktopScrollbarOverlay({
         ]}
         pointerEvents={handleVisible ? "auto" : "none"}
         {...(panResponder?.panHandlers ?? {})}
-        {...(isWeb
+        {...(platformIsWeb
           ? ({
               onPointerDown: startWebDrag,
               onPointerEnter: handleGrabHoverIn,
@@ -403,7 +402,7 @@ export function WebDesktopScrollbarOverlay({
               backgroundColor: handleColor,
               opacity: handleOpacity,
             },
-            isWeb &&
+            platformIsWeb &&
               ({
                 transitionProperty: "opacity, width, background-color",
                 transitionDuration: `${HANDLE_FADE_DURATION_MS}ms, ${HANDLE_WIDTH_TRANSITION_DURATION_MS}ms, ${HANDLE_FADE_DURATION_MS}ms`,