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.

crates/tauri-plugin-storage/src/lib.rs

@@ -33,6 +33,12 @@ pub struct AccessState {
     pub granted: bool,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
+#[serde(rename_all = "camelCase")]
+pub struct PickedImage {
+    pub path: Option<String>,
+}
+
 #[cfg(mobile)]
 pub struct Storage<R: Runtime>(tauri::plugin::PluginHandle<R>);
 
@@ -91,9 +97,30 @@ fn request_all_files_access<R: Runtime>(app: tauri::AppHandle<R>) -> Result<(),
     }
 }
 
+/// A photo from the phone: the document picker limited to images, the
+/// picked content copied into the app's cache — a plain path for the Rust
+/// side (a playlist's cover, for one).
+#[tauri::command]
+async fn pick_image<R: Runtime>(app: tauri::AppHandle<R>) -> Result<PickedImage, String> {
+    #[cfg(mobile)]
+    {
+        use tauri::Manager;
+        let storage = app.state::<Storage<R>>();
+        storage
+            .0
+            .run_mobile_plugin::<PickedImage>("pickImage", ())
+            .map_err(|e| e.to_string())
+    }
+    #[cfg(not(mobile))]
+    {
+        let _ = app;
+        Err("the image picker here is the desktop's dialog".into())
+    }
+}
+
 pub fn init<R: Runtime>() -> TauriPlugin<R> {
     Builder::new("storage")
-        .invoke_handler(tauri::generate_handler![pick_folder, all_files_access, request_all_files_access])
+        .invoke_handler(tauri::generate_handler![pick_folder, all_files_access, request_all_files_access, pick_image])
         .setup(|app, api| {
             #[cfg(target_os = "android")]
             {