feat(app): native terminal on WebView with xterm.js

Replace the Expo DOM-backed terminal on iOS and Android with a managed
WebView running xterm.js. Web and desktop continue to use the existing
DOM implementation.

Mobile mounted tabs switch from display:none to opacity:0 for the hidden
slot on purpose: terminals stay mounted under the LRU tab cache, and
keeping the WebView in the layer tree avoids cold-starting it every time
the user returns to a terminal tab.

EAS native builds rebuild the WebView bundle post-install so the
generated HTML stays in sync with the entry source.

This does not fix the WS 1006 / NSPOSIXErrorDomain Code=54 host-disconnect
symptom that prompted the investigation. After restoring the baseline,
that symptom could not be replicated; do not read this commit as its
root cause or fix.

packages/app/src/screens/workspace/workspace-screen.tsx

@@ -714,13 +714,12 @@ const MobileMountedTabSlot = memo(function MobileMountedTabSlot({
     [buildPaneContentModel, paneId, tabDescriptor],
   );
 
-  const slotStyle = useMemo(
-    () => ({ display: isVisible ? ("flex" as const) : ("none" as const), flex: 1 }),
-    [isVisible],
-  );
+  const slotStyle = isVisible
+    ? styles.mobileMountedTabSlotVisible
+    : styles.mobileMountedTabSlotHidden;
 
   return (
-    <View style={slotStyle}>
+    <View style={slotStyle} pointerEvents={isVisible ? "auto" : "none"}>
       <WorkspacePaneContent
         content={content}
         isWorkspaceFocused={isWorkspaceFocused}
@@ -2854,7 +2853,7 @@ function WorkspaceScreenContent({
   const focusedPaneTabIds = useMemo(() => tabs.map((tab) => tab.tabId), [tabs]);
   const focusedPaneTabDescriptorMap = useStableTabDescriptorMap(tabs);
   const { mountedTabIds: mountedFocusedPaneTabIdsSet } = useMountedTabSet({
-    activeTabId: activeTabDescriptor?.tabId ?? null,
+    activeTabId,
     allTabIds: focusedPaneTabIds,
     cap: 3,
   });
@@ -3654,6 +3653,15 @@ const styles = StyleSheet.create((theme) => ({
     flex: 1,
     minHeight: 0,
     backgroundColor: theme.colors.surface0,
+    position: "relative",
+  },
+  mobileMountedTabSlotVisible: {
+    ...StyleSheet.absoluteFillObject,
+    opacity: 1,
+  },
+  mobileMountedTabSlotHidden: {
+    ...StyleSheet.absoluteFillObject,
+    opacity: 0,
   },
   contentPlaceholder: {
     flex: 1,