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/ui/combobox.tsx

@@ -37,8 +37,9 @@ import {
   shouldShowCustomComboboxOption,
 } from "./combobox-options";
 import type { ComboboxOptionModel } from "./combobox-options";
+import { isWeb } from "@/constants/platform";
 
-const IS_WEB = Platform.OS === "web";
+const IS_WEB = isWeb;
 
 export type ComboboxOption = ComboboxOptionModel;
 
@@ -107,6 +108,7 @@ export interface SearchInputProps {
   onChangeText: (text: string) => void;
   onSubmitEditing?: () => void;
   autoFocus?: boolean;
+  useBottomSheetInput?: boolean;
 }
 
 export function SearchInput({
@@ -115,10 +117,11 @@ export function SearchInput({
   onChangeText,
   onSubmitEditing,
   autoFocus = false,
+  useBottomSheetInput = false,
 }: SearchInputProps): ReactElement {
   const { theme } = useUnistyles();
   const inputRef = useRef<TextInput>(null);
-  const InputComponent = Platform.OS === "web" ? TextInput : BottomSheetTextInput;
+  const InputComponent = useBottomSheetInput ? BottomSheetTextInput : TextInput;
 
   useEffect(() => {
     if (autoFocus && IS_WEB && inputRef.current) {
@@ -264,8 +267,7 @@ export function Combobox({
 }: ComboboxProps): ReactElement {
   const isMobile = useIsCompactFormFactor();
   const effectiveOptionsPosition = isMobile ? "below-search" : optionsPosition;
-  const isDesktopAboveSearch =
-    !isMobile && Platform.OS === "web" && effectiveOptionsPosition === "above-search";
+  const isDesktopAboveSearch = !isMobile && isWeb && effectiveOptionsPosition === "above-search";
   const { height: windowHeight } = useWindowDimensions();
   const bottomSheetRef = useRef<BottomSheetModal>(null);
   const hasPresentedBottomSheetRef = useRef(false);
@@ -322,8 +324,8 @@ export function Combobox({
 
   const middleware = useMemo(
     () => [
-      floatingOffset(Platform.OS === "web" ? 0 : 4),
-      ...(Platform.OS === "web" ? [] : [flip({ padding: collisionPadding })]),
+      floatingOffset(isWeb ? 0 : 4),
+      ...(isWeb ? [] : [flip({ padding: collisionPadding })]),
       ...(isDesktopAboveSearch ? [] : [shift({ padding: collisionPadding })]),
       floatingSize({
         padding: collisionPadding,
@@ -346,7 +348,7 @@ export function Combobox({
   );
 
   const { refs, floatingStyles, update } = useFloating({
-    placement: Platform.OS === "web" ? desktopPlacement : "bottom-start",
+    placement: isWeb ? desktopPlacement : "bottom-start",
     middleware,
     sameScrollView: false,
     elements: {
@@ -626,6 +628,7 @@ export function Combobox({
       onChangeText={setSearchQueryWithCallback}
       onSubmitEditing={handleSubmitSearch}
       autoFocus={!isMobile}
+      useBottomSheetInput={isMobile}
     />
   );