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/android/src/main/java/com/soulstream/storage/StoragePlugin.kt

@@ -91,4 +91,37 @@ class StoragePlugin(private val activity: Activity) : Plugin(activity) {
         }
         invoke.resolve()
     }
+    /** A photo from the phone: the document picker limited to images, the
+     *  picked content copied into the app's cache so the Rust side gets a
+     *  plain path (a content URI is not one). */
+    @Command
+    fun pickImage(invoke: Invoke) {
+        val intent = Intent(Intent.ACTION_OPEN_DOCUMENT)
+        intent.addCategory(Intent.CATEGORY_OPENABLE)
+        intent.type = "image/*"
+        startActivityForResult(invoke, intent, "imagePicked")
+    }
+
+    @ActivityCallback
+    fun imagePicked(invoke: Invoke, result: ActivityResult) {
+        when (result.resultCode) {
+            Activity.RESULT_OK -> {
+                val uri = result.data?.data
+                if (uri == null) { invoke.reject("no image came back"); return }
+                try {
+                    val target = java.io.File(activity.cacheDir, "picked-cover-" + System.currentTimeMillis() + ".img")
+                    val input = activity.contentResolver.openInputStream(uri)
+                    if (input == null) { invoke.reject("the image could not be read"); return }
+                    input.use { i -> target.outputStream().use { o -> i.copyTo(o) } }
+                    val out = JSObject()
+                    out.put("path", target.absolutePath)
+                    invoke.resolve(out)
+                } catch (e: Exception) {
+                    invoke.reject("the image could not be read: " + e.message)
+                }
+            }
+            Activity.RESULT_CANCELED -> invoke.reject("cancelled")
+            else -> invoke.reject("the picker failed")
+        }
+    }
 }