android: a folder of audio files, and a paste of track links

The paste door now reads itself the way the desktop's does: one track link
in the paste and every line is looked up on its own public page
(playlist_import_track_links), which is the way past Spotify's 100-track
public ceiling — the wait screen counts them as they arrive. A fourth door
imports a folder of audio files off this phone, picked with the storage
plugin's own document-tree picker; playlist_import_folder grew a path
argument for it, because on Android it had no dialog to open and always
answered nothing. A YouTube playlist link needed no door at all — the link
one already dispatches it.

src-tauri/src/playlists.rs

@@ -721,20 +721,31 @@ pub async fn playlist_import_probe(
 pub async fn playlist_import_folder(
     app: tauri::AppHandle,
     lib: State<'_, LibraryState>,
+    path: Option<String>,
 ) -> Result<Option<ImportDraft>, String> {
-    let (tx, rx) = tokio::sync::oneshot::channel::<Option<String>>();
-    #[cfg(desktop)]
-    {
-        use tauri_plugin_dialog::DialogExt;
-        app.dialog().file().pick_folder(move |path| {
-            let _ = tx.send(path.map(|p| p.to_string()));
-        });
-    }
-    #[cfg(mobile)]
-    {
-        let _ = tx.send(None);
-    }
-    let Some(root) = rx.await.unwrap_or(None) else {
+    // The phone has no dialog here, so it picks the folder itself (the
+    // storage plugin's document-tree picker) and hands the resolved path
+    // in; the desktop passes nothing and opens its own.
+    let picked = match path {
+        Some(p) if !p.trim().is_empty() => Some(p),
+        _ => {
+            let (tx, rx) = tokio::sync::oneshot::channel::<Option<String>>();
+            #[cfg(desktop)]
+            {
+                use tauri_plugin_dialog::DialogExt;
+                app.dialog().file().pick_folder(move |path| {
+                    let _ = tx.send(path.map(|p| p.to_string()));
+                });
+            }
+            #[cfg(mobile)]
+            {
+                let _ = &app;
+                let _ = tx.send(None);
+            }
+            rx.await.unwrap_or(None)
+        }
+    };
+    let Some(root) = picked else {
         return Ok(None);
     };
     let root_path = PathBuf::from(&root);