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,