iOS thumbnails: batch pre-fetch after photo listing

After loadPhotos completes for an iPhone, spawn a background coroutine
that runs batch-thumbs in chunks of 30 photos per Python process call.
One Python process handles 30 thumbnails instead of one per photo —
reduces startup overhead by 30x.

sips resizes each pulled file to 240px and writes to the cache.
DenseThumb then reads from cache instantly. Files already cached are
skipped.

src/main/kotlin/com/bolenpad/photophetch/device/IosConnector.kt

@@ -42,7 +42,7 @@ class IosConnector(
     private val log = LoggerFactory.getLogger(IosConnector::class.java)
     private val json = Json { ignoreUnknownKeys = true }
 
-    private val pythonBin: String = pythonBinOverride ?: discoverPython()
+    internal val pythonBin: String = pythonBinOverride ?: discoverPython()
 
     companion object {
         private val PIPX_PYTHON_CANDIDATES = listOf(
@@ -67,7 +67,7 @@ class IosConnector(
     }
 
     /** Path to the bundled helper script, extracted to a temp location on first use */
-    private val helperScript: String by lazy { extractHelperScript() }
+    internal val helperScript: String by lazy { extractHelperScript() }
 
     override suspend fun detectDevices(): List<DeviceInfo> {
         return try {

src/main/kotlin/com/bolenpad/photophetch/ui/AppState.kt

@@ -157,6 +157,13 @@ class AppState(private val configStore: ConfigStore) {
                 // Default: select the first camera-roll folder
                 val defaultFolder = folders.firstOrNull { it.isCameraRoll }?.name
                 selectFolder(defaultFolder)
+
+                // For iOS: batch pre-fetch thumbnails in background (amortises Python startup cost)
+                if (device.type == DeviceType.IOS && connector is com.bolenpad.photophetch.device.IosConnector) {
+                    launch(Dispatchers.IO) {
+                        prefetchIosThumbnails(device, raw, connector)
+                    }
+                }
             } catch (e: Exception) {
                 log.error("Photo listing failed", e)
                 _photoList.value = PhotoListState.Error(e.message ?: "Unknown error")
@@ -164,6 +171,71 @@ class AppState(private val configStore: ConfigStore) {
         }
     }
 
+    private suspend fun prefetchIosThumbnails(
+        device: DeviceInfo,
+        photos: List<PhonePhoto>,
+        connector: com.bolenpad.photophetch.device.IosConnector,
+    ) {
+        val cacheKey = connector.cacheKey(device)
+        val cacheRoot = java.nio.file.Paths.get(System.getProperty("user.home"), ".photophetch", "thumbcache")
+        val cacheDir = cacheRoot.resolve(cacheKey.replace("/", "_").replace(":", "_"))
+        if (!cacheDir.toFile().exists()) cacheDir.toFile().mkdirs()
+        val tmpDir = java.nio.file.Paths.get(System.getProperty("java.io.tmpdir"), "photophetch-thumbs")
+        if (!tmpDir.toFile().exists()) tmpDir.toFile().mkdirs()
+
+        // Only fetch photos not already cached
+        val needed = photos.filter { photo ->
+            val ext = photo.filename.substringAfterLast(".", "").lowercase()
+            if (ext in setOf("mp4", "mov", "3gp", "avi", "mkv", "m4v", "webm")) return@filter false
+            val hash = (cacheKey + photo.devicePath).hashCode().toString(16)
+            val cachePath = cacheDir.resolve("$hash.jpg")
+            !cachePath.toFile().exists() || cachePath.toFile().length() == 0L
+        }
+
+        if (needed.isEmpty()) return
+        log.info("Pre-fetching ${needed.size} iOS thumbnails...")
+
+        // Build pairs JSON and run batch-thumbs command
+        needed.chunked(30).forEach { batch ->
+            val pairs = batch.joinToString(",") { photo ->
+                val ext = photo.filename.substringAfterLast(".", "").lowercase()
+                val hash = (cacheKey + photo.devicePath).hashCode().toString(16)
+                val tmpPath = tmpDir.resolve("$hash.$ext")
+                val src = photo.devicePath.replace("\"", "\\\"")
+                val dst = tmpPath.toString().replace("\"", "\\\"")
+                "[\"$src\",\"$dst\"]"
+            }
+            val pairsJson = "[$pairs]"
+
+            try {
+                val process = ProcessBuilder(connector.pythonBin, connector.helperScript, "batch-thumbs", pairsJson)
+                    .redirectErrorStream(false).start()
+                process.inputStream.readBytes() // stdout
+                process.errorStream.readBytes() // stderr
+                process.waitFor()
+
+                // Run sips on each successfully pulled file
+                batch.forEach { photo ->
+                    val ext = photo.filename.substringAfterLast(".", "").lowercase()
+                    val hash = (cacheKey + photo.devicePath).hashCode().toString(16)
+                    val tmpFile = tmpDir.resolve("$hash.$ext")
+                    val cachePath = cacheDir.resolve("$hash.jpg")
+                    if (tmpFile.toFile().exists() && tmpFile.toFile().length() > 0 && !cachePath.toFile().exists()) {
+                        val sips = ProcessBuilder("sips", "--resampleWidth", "240",
+                            tmpFile.toString(), "--out", cachePath.toString())
+                            .redirectErrorStream(true).start()
+                        sips.inputStream.readBytes()
+                        sips.waitFor()
+                        tmpFile.toFile().delete()
+                    }
+                }
+            } catch (e: Exception) {
+                log.warn("iOS thumbnail batch failed: ${e.message}")
+            }
+        }
+        log.info("iOS thumbnail pre-fetch complete")
+    }
+
     fun selectFolder(folderName: String?) {
         _selectedFolder.value = folderName
         _selectedPhotos.value = emptySet()