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/build.rs

@@ -1,6 +1,6 @@
 // The three commands, as the Rust functions are named; the ACL slugs them
 // (`allow-pick-folder`, …) into permissions/autogenerated.
-const COMMANDS: &[&str] = &["pick_folder", "all_files_access", "request_all_files_access"];
+const COMMANDS: &[&str] = &["pick_folder", "all_files_access", "request_all_files_access", "pick_image"];
 
 fn main() {
     // Registers the command permissions and, on Android, hands the Gradle