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/permissions/autogenerated/reference.md

@@ -9,6 +9,7 @@ whether it is granted, and the system screen that grants it.
 - `allow-pick-folder`
 - `allow-all-files-access`
 - `allow-request-all-files-access`
+- `allow-pick-image`
 
 ## Permission Table
 
@@ -74,6 +75,32 @@ Denies the pick_folder command without any pre-configured scope.
 <tr>
 <td>
 
+`storage:allow-pick-image`
+
+</td>
+<td>
+
+Enables the pick_image command without any pre-configured scope.
+
+</td>
+</tr>
+
+<tr>
+<td>
+
+`storage:deny-pick-image`
+
+</td>
+<td>
+
+Denies the pick_image command without any pre-configured scope.
+
+</td>
+</tr>
+
+<tr>
+<td>
+
 `storage:allow-request-all-files-access`
 
 </td>