Fix explorer back and session usage selection

app/androidApp/src/main/kotlin/com/example/aiapp/FilesScreen.kt

@@ -61,9 +61,8 @@ private sealed class Spot(val path: String) {
  * The files on the machine a session runs on: browse them, read one, change one.
  *
  * Drawn **over** the session rather than instead of it (see [AppRoot]), so its event stream keeps
- * flowing and coming back from a file costs nothing. Back steps one level inside here -- editor to
- * viewer, viewer to the directory it came from, directory to the one above -- and only closes from
- * where it opened.
+ * flowing and coming back from a file costs nothing. Back always closes the explorer and returns to
+ * that session; directory navigation stays inside the listing, where its `..` row is explicit.
  *
  * Every directory that has been visited is kept for as long as this is open; the refresh glyph is
  * how one gets asked again on purpose, and creating something refetches the directory it was
@@ -72,7 +71,7 @@ private sealed class Spot(val path: String) {
 @Composable
 fun FilesScreen(settings: ServerSettings, target: FilesTarget, onClose: () -> Unit) {
     val scope = rememberCoroutineScope()
-    var stack by remember { mutableStateOf(listOf<Spot>(Spot.Dir(target.start))) }
+    var here by remember { mutableStateOf<Spot>(Spot.Dir(target.start)) }
     val listings = remember { mutableStateMapOf<String, LoadState<Listing>>() }
     var creating by remember { mutableStateOf(false) }
     // Edit mode and whether anything has been typed live here rather than in the pane below,
@@ -82,25 +81,14 @@ fun FilesScreen(settings: ServerSettings, target: FilesTarget, onClose: () -> Un
     var dirty by remember { mutableStateOf(false) }
     var askUnsaved by remember { mutableStateOf(false) }
 
-    val here = stack.last()
-
     fun go(spot: Spot) {
         editing = false
         dirty = false
-        stack = stack + spot
+        here = spot
     }
 
     fun back() {
-        when {
-            editing && dirty -> askUnsaved = true
-            editing -> editing = false
-            stack.size > 1 -> {
-                stack = stack.dropLast(1)
-                editing = false
-                dirty = false
-            }
-            else -> onClose()
-        }
+        if (editing && dirty) askUnsaved = true else onClose()
     }
 
     suspend fun load(path: String, again: Boolean) {
@@ -173,8 +161,7 @@ fun FilesScreen(settings: ServerSettings, target: FilesTarget, onClose: () -> Un
         UnsavedDialog(
             onDiscard = {
                 askUnsaved = false
-                editing = false
-                dirty = false
+                onClose()
             },
             onCancel = { askUnsaved = false },
         )