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/default.toml

@@ -6,4 +6,4 @@ The phone's storage: pick a library folder (a Storage Access Framework tree,
 resolved to a path when the volume allows), and Android's all-files access —
 whether it is granted, and the system screen that grants it.
 """
-permissions = ["allow-pick-folder", "allow-all-files-access", "allow-request-all-files-access"]
+permissions = ["allow-pick-folder", "allow-all-files-access", "allow-request-all-files-access", "allow-pick-image"]