Reduce iOS memory pressure: lightbox windowing, memory warning handler

- Lightbox pager: only mount ±1 neighbor of the current page so a
  multi-image post no longer decodes every fullsize image up front.
- App: respond to iOS memoryWarning events by clearing expo-image's
  in-memory cache, giving the GC a stronger hint before jetsam.
- GIF picker: priority="low" + recyclingKey on item Image. Minor; the
  picker's masonry grid still needs proper row virtualization.

Hypothesis-driven against APP-D9 breadcrumb patterns; needs TestFlight
validation before claiming any rate reduction.

src/App.tsx

@@ -1,12 +1,14 @@
 import '#/logger/sentry/setup'
 
 import {Fragment, useEffect, useState} from 'react'
+import {DeviceEventEmitter} from 'react-native'
 import {GestureHandlerRootView} from 'react-native-gesture-handler'
 import {KeyboardProvider as KeyboardControllerProvider} from 'react-native-keyboard-controller'
 import {
   initialWindowMetrics,
   SafeAreaProvider,
 } from 'react-native-safe-area-context'
+import {Image} from 'expo-image'
 import * as ScreenOrientation from 'expo-screen-orientation'
 import * as SplashScreen from 'expo-splash-screen'
 import * as SystemUI from 'expo-system-ui'
@@ -141,6 +143,16 @@ function InnerApp() {
     })
   }, [l])
 
+  // Respond to iOS memory warnings before the OS jetsams us. expo-image trims
+  // its own cache on warning, but explicitly clearing it gives the GC a stronger
+  // hint and frees decoded bitmaps no longer in active use.
+  useEffect(() => {
+    const sub = DeviceEventEmitter.addListener('memoryWarning', () => {
+      void Image.clearMemoryCache()
+    })
+    return () => sub.remove()
+  }, [])
+
   return (
     <Alf theme={theme}>
       <ThemeProvider theme={theme}>

src/components/Lightbox/pager/ImagePager.tsx

@@ -415,6 +415,7 @@ function ImageView({
         style={styles.pager}>
         {images.map((imageSrc, i) => (
           <View key={`${i}-${imageSrc.uri}`}>
+            {Math.abs(i - imageIndex) <= 1 ? (
               <LightboxImage
                 onTap={onTap}
                 onZoom={onZoom}
@@ -431,6 +432,7 @@ function ImageView({
                 thumbRects={thumbRects}
                 imageIndex={i}
               />
+            ) : null}
           </View>
         ))}
       </PagerView>

src/features/gifPicker/components/GifPickerItem.tsx

@@ -52,6 +52,8 @@ export function GifPickerItem({
           accessibilityLabel={gif.title}
           accessibilityHint=""
           cachePolicy="none"
+          priority="low"
+          recyclingKey={gif.id}
           accessibilityIgnoresInvertColors
         />
       )}