Document and handle USB conflict between iPhone and Android in Charging mode

IosConnector: add comment explaining 0-folders causes.
DensePhotoGrid: show actionable hint when no photos found (unlock screen,
switch Android to File Transfer, killall PTPCamera).
README: add 'Using iPhone and Android at the same time' section.

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

@@ -94,6 +94,15 @@ class IosConnector(
             rawPhotos.add(photo)
         }
 
+        if (rawPhotos.isEmpty()) {
+            // gphoto2 connected but found no files. Common causes:
+            // 1. iPhone screen is locked — unlock and retry
+            // 2. Another USB device (e.g. Android phone in Charging mode) is blocking
+            //    USB enumeration — unplug other devices or switch Android to File Transfer mode
+            // 3. PTPCamera daemon grabbed the device — run: killall PTPCamera
+            // We return empty and let the UI show the no-photos state with a hint.
+        }
+
         // Detect Live Photo pairs: HEIC + MOV with the same base name and close timestamps
         return tagLivePhotoPairs(rawPhotos)
     }

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

@@ -51,8 +51,22 @@ fun DensePhotoGrid(
     modifier: Modifier = Modifier,
 ) {
     if (folderSections.isEmpty() || folderSections.all { it.allPhotos.isEmpty() }) {
+        val isIos = folderSections.isEmpty() // IosConnector returns no folders when blocked
         Box(modifier.fillMaxSize(), contentAlignment = Alignment.Center) {
-            Text("No photos", color = MaterialTheme.colorScheme.onSurfaceVariant)
+            Column(
+                horizontalAlignment = Alignment.CenterHorizontally,
+                verticalArrangement = Arrangement.spacedBy(8.dp),
+                modifier = Modifier.padding(32.dp),
+            ) {
+                Text("No photos found", style = MaterialTheme.typography.titleMedium)
+                Text(
+                    "• Make sure the iPhone screen is unlocked\n" +
+                    "• If another phone is plugged in, switch it to \"Transferring files\" mode\n" +
+                    "• Run: killall PTPCamera",
+                    style = MaterialTheme.typography.bodySmall,
+                    color = MaterialTheme.colorScheme.onSurfaceVariant,
+                )
+            }
         }
         return
     }