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/HalfSheet.svelte

@@ -12,7 +12,7 @@
     import { player, playNext, enqueue, playEntries, playQueueAt, removeFromQueue, moveToNext, moveInQueue, setSleep, sleepLabel } from "./player.svelte.js";
     import { streamSong, saveSong, streamCatalogAlbum, saveCatalogAlbum } from "./resolve.svelte.js";
     import { catalog, yearOf } from "./catalog.svelte.js";
-    import { pls, bulk, bulkable, runBulk, coverFor, queueable, entryOf, playFrom, shufflePlay, togglePin, renamePlaylist, describePlaylist, duplicatePlaylist, deletePlaylist, removeItems, addEntries, createPlaylist, moveItem } from "./playlists.svelte.js";
+    import { pls, bulk, bulkable, runBulk, coverFor, queueable, entryOf, playFrom, shufflePlay, togglePin, renamePlaylist, describePlaylist, duplicatePlaylist, deletePlaylist, removeItems, addEntries, createPlaylist, moveItem, coverFromPhoto, coverClear } from "./playlists.svelte.js";
     import { DOORS, start as startImport } from "./importer.svelte.js";
     import { slsk, refreshRooms, disconnect, openThread, addBuddy, removeBuddy, leaveRoom } from "./slsk.svelte.js";
     import { lib, albumById, albumsBy, tracksOf, loadLibrary, refreshPlaylists, refreshRecents } from "./lib.svelte.js";
@@ -374,6 +374,11 @@
                     {@render row("file-text", p.description ? "Edit description" : "Add a description", p.description || "", () => describePrompt(p))}
                 {/if}
                 {@render row("layers", "Duplicate", "a copy, “" + p.name + " copy”", act(() => duplicatePlaylist(p).then((id) => push("playlist", { id })).catch(oops)))}
+                <div class="sheet-hair"></div>
+                {@render row("image", "Choose cover photo…", "a picture from this phone", act(() => coverFromPhoto(p).catch(oops)))}
+                {#if p.cover_path}
+                    {@render row("x", "Remove custom cover", "the mosaic comes back", act(() => coverClear(p).catch(oops)))}
+                {/if}
                 {#if kind === "playlist-more" && unres.length}
                     {@render row("x", "Remove unresolved entries", unres.length + " that never matched a track", act(() => removeItems(p, rows, unres).catch(oops)))}
                 {/if}