android: a playlist's cover from a photo on the phone

The storage plugin gains pick_image — the document picker limited to
images, the picked content copied into the app's cache so the Rust side
gets a plain path. playlist_cover_set(id, path) is the desktop pick's
install step split out and shared (squared, the master cover, the row's
revision moved). A playlist's sheet offers Choose cover photo and Remove
custom cover; the mock walk exercises both.

src-tauri/src/lib.rs

@@ -259,6 +259,7 @@ pub fn run() {
             playlists::playlist_relink_item,
             playlists::playlist_cover,
             playlists::playlist_cover_pick,
+            playlists::playlist_cover_set,
             playlists::playlist_cover_clear,
             playlists::playlist_import_probe,
             playlists::playlist_import_folder,

src-tauri/src/playlists.rs

@@ -399,7 +399,18 @@ pub async fn playlist_cover_pick(
     let Some(source) = rx.await.unwrap_or(None) else {
         return Ok(None);
     };
-    let dir = covers_dir(&app)?;
+    install_cover(&app, &lib, id, source).await.map(Some)
+}
+
+/// A picked image — the desktop's dialog, or a photo the phone copied into
+/// its cache — squared, written as the playlist's master cover, recorded.
+async fn install_cover(
+    app: &tauri::AppHandle,
+    lib: &LibraryState,
+    id: i64,
+    source: String,
+) -> Result<String, String> {
+    let dir = covers_dir(app)?;
     let target = dir.join(format!("{id}.jpg"));
     let jpeg = tauri::async_runtime::spawn_blocking(move || -> Result<Vec<u8>, String> {
         let bytes = std::fs::read(&source).map_err(|e| e.to_string())?;
@@ -422,8 +433,20 @@ pub async fn playlist_cover_pick(
     .await
     .map_err(|e| e.to_string())?
     .map_err(|e| e.to_string())?;
-    changed(&app);
-    Ok(Some(data_url_jpeg(&jpeg)))
+    changed(app);
+    Ok(data_url_jpeg(&jpeg))
+}
+
+/// The phone's door: a photo already on disk (the storage plugin copied it
+/// into the app's cache) becomes the playlist's cover.
+#[tauri::command]
+pub async fn playlist_cover_set(
+    app: tauri::AppHandle,
+    lib: State<'_, LibraryState>,
+    id: i64,
+    path: String,
+) -> Result<String, String> {
+    install_cover(&app, &lib, id, path).await
 }
 
 #[tauri::command]