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/commands/pick_image.toml

@@ -0,0 +1,13 @@
+# Automatically generated - DO NOT EDIT!
+
+"$schema" = "../../schemas/schema.json"
+
+[[permission]]
+identifier = "allow-pick-image"
+description = "Enables the pick_image command without any pre-configured scope."
+commands.allow = ["pick_image"]
+
+[[permission]]
+identifier = "deny-pick-image"
+description = "Denies the pick_image command without any pre-configured scope."
+commands.deny = ["pick_image"]