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.

ANDROID.md

@@ -268,6 +268,11 @@ describe / pin / duplicate / delete, `playlist_add_items`,
   byte-complete, the second finds a source for every stream row (the
   remembered one, else the ladder) and hands it to the download pipeline;
   the page shows the run's line with Stop, then its summary.
+- **The cover**: Choose cover photo on a playlist's sheet opens the phone's
+  image picker (the storage plugin's `pick_image`: the picked content is
+  copied into the app's cache, and `playlist_cover_set` installs it the way
+  the desktop's dialog pick does — squared, the master cover, the row's
+  revision moved); Remove custom cover brings the mosaic back.
 - **Add to playlist** is on every track and record sheet: a pick-a-playlist
   sheet (New playlist at the top). A file on the phone becomes a local row;
   a streamed or catalog track keeps its identity (`track_id`) and cover URL
@@ -545,7 +550,7 @@ paused) without the walk.
   is Kyle's to join.
 - The desktop-only doors and tools: the file and folder playlist imports,
   the listener door and the lyric alignment models (a desktop runtime),
-  playlist export and the cover picker, drag reorder, catalog track
+  playlist export, drag reorder (Move up / down / to the top stand in), catalog track
   previews, the full discography page, the artist-artwork upscaler,
   updates and the account section (sign-in lives on Community).
 - Follows and the play history are per library folder, so the phone's own