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/mobile/playlists.svelte.js

@@ -184,6 +184,22 @@ export async function deletePlaylist(p) {
     await refreshPlaylists();
 }
 
+/* ── the cover: a photo from the phone, or the mosaic again ── */
+export async function coverFromPhoto(p) {
+    let r;
+    try { r = await api.pickImage(); } catch (e) { if (/cancelled/.test(String(e))) return false; throw e; }
+    if (!r?.path) return false;
+    await api.playlistCoverSet(p.id, r.path);
+    await refreshPlaylists();
+    note(`Cover set for ${p.name}`);
+    return true;
+}
+export async function coverClear(p) {
+    await api.playlistCoverClear(p.id);
+    await refreshPlaylists();
+    note(`Back to the mosaic for ${p.name}`);
+}
+
 /* ── reorder: a row up, down or to the top; the complete id order goes
    in one call, as the desktop's drop does ── */
 export async function moveItem(p, items, item, where) {