Stop promising to ask for an export folder

An empty device destination read "Ask each time" on the settings page,
and nothing asks: an export made with the field blank is refused with
"no export folder is set". Say what will happen.

core/dr-types/src/settings.rs

@@ -885,9 +885,14 @@ impl ExportSettings {
     ///
     /// The library root has to read as a place rather than as a blank field,
     /// or confirming the picker where it opens looks like it did nothing.
+    ///
+    /// An empty device folder used to read "Ask each time", which nothing
+    /// does: no platform asks, and an export made with the field blank is
+    /// refused with "no export folder is set". The label now says what will
+    /// happen rather than what was once meant to.
     pub fn destination_label(&self) -> &str {
         match self.target {
-            ExportTarget::Device if self.destination.trim().is_empty() => "Ask each time",
+            ExportTarget::Device if self.destination.trim().is_empty() => "Not set",
             ExportTarget::Device => &self.destination,
             ExportTarget::Remote if self.remote_destination.trim().is_empty() => "Library root",
             ExportTarget::Remote => &self.remote_destination,
@@ -1621,14 +1626,15 @@ mod tests {
     }
 
     #[test]
-    fn an_empty_device_destination_still_means_ask() {
+    fn an_empty_device_destination_is_not_set_and_says_so() {
         // The asymmetry is deliberate: a filesystem has no folder worth
-        // assuming, so empty there is a question rather than an answer.
+        // assuming, so empty there is a gap rather than an answer — and the
+        // label must not promise a question nobody will be asked.
         let mut s = Settings::default();
         s.export.target = ExportTarget::Device;
         s.export.destination = String::new();
         assert!(!s.export.destination_is_set());
-        assert_eq!(s.export.destination_label(), "Ask each time");
+        assert_eq!(s.export.destination_label(), "Not set");
     }
 
     #[test]

ui/dr-ui/ui/settings.slint

@@ -1161,7 +1161,10 @@ export component SettingsPage inherits Rectangle {
 
                         TextRow {
                             label: "Destination";
-                            hint: "empty asks each time";
+                            // What an empty field does, not what it was once
+                            // going to do: nothing asks, and an export with
+                            // no folder is refused and says so in the header.
+                            hint: "a folder on this device; exports are refused until one is set";
                             text <=> root.destination;
                             field-width: 320px;
                             placeholder: root.destination-hint;