android: a playlist, out of the phone

The desktop has had Export since playlists v2 and the phone had no door onto
it for one honest reason: `tauri-plugin-dialog` does open Android's
create-document sheet, but it answers with a `content://` URI and
`playlist_export` wrote with `std::fs::write`, which cannot take one. The
storage plugin grew a `create_document` whose Kotlin writes the bytes through
the ContentResolver — a plugin function called from Rust, not a command, so
the file never passes through the WebView and nothing was added to the ACL —
and `playlist_export` now builds the file above the platform fork with the
same `playlist_formats` writers, so the phone rebuilds no format and the two
exports cannot drift. The door is the playlist's own sheet, on the page and
on the shelf card, with the desktop's five profiles in the desktop's order; a
dismissed sheet says nothing, because nothing happened. The mock walk drives
it three ways — a file written where the sheet pointed, a sheet dismissed, a
refusal read back as a sentence — and the phone walk opens the real sheet and
dismisses it with BACK, writing nothing to the phone. The Kotlin is unbuilt:
nothing in this tree compiles it, though it is written against
tauri-android's own `PluginHandle`, `Invoke` and `JSObject` rather than
against a guess at them.

src/mobile/HalfSheet.svelte

@@ -13,7 +13,7 @@
     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, artistKey, picture, heroFromPhoto, heroClear } from "./catalog.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 { pls, bulk, bulkable, runBulk, coverFor, queueable, entryOf, playFrom, shufflePlay, togglePin, renamePlaylist, describePlaylist, duplicatePlaylist, deletePlaylist, removeItems, addEntries, createPlaylist, moveItem, coverFromPhoto, coverClear, EXPORTS, exportPlaylist } from "./playlists.svelte.js";
     import { DOORS, start as startImport, pickImportFolder, startFolder } from "./importer.svelte.js";
     import { slsk, refreshRooms, disconnect, openThread, addBuddy, removeBuddy, leaveRoom } from "./slsk.svelte.js";
     import { lib, albumById, tracksOf, loadLibrary, refreshPlaylists } from "./lib.svelte.js";
@@ -475,6 +475,8 @@
                     {@render row("download", "Download all tracks", bulkable(rows) + " not in the library yet, by your download rules", act(() => { void runBulk(p, rows, "download"); }))}
                 {/if}
                 <div class="sheet-hair"></div>
+                {@render row("export", "Export…", p.track_count + " track" + (p.track_count === 1 ? "" : "s") + ", as a playlist file", () => openHalf("plexport", { playlist: p }))}
+                <div class="sheet-hair"></div>
                 {@render row("trash", "Delete playlist", "", () => confirmDelete(p))}
             </div>
         {:else if kind === "pltrack" && shown?.arg?.item}
@@ -812,6 +814,15 @@
                 <div class="setup-foot"><button class="btn btn-fill wide" onclick={() => void finishSetup()}>Start listening</button></div>
                 <div class="sec-note" style="padding-bottom:16px">Nothing above is required. You → Set up this phone again opens it back up.</div>
             </div>
+        {:else if kind === "plexport" && shown?.arg?.playlist}
+            {@const p = shown.arg.playlist}
+            <div class="sheet-title">Export <span class="t-sub">· {p.name}</span></div>
+            <div class="sheet-rows">
+                {#each EXPORTS as [format, label, m] (format)}
+                    {@render row("export", label, m, act(() => exportPlaylist(p, format).catch(oops)))}
+                {/each}
+            </div>
+            <div class="sec-note" style="padding-bottom:16px">The file goes wherever the sheet points it — Downloads, Drive, anywhere the phone offers. A row with no file of its own keeps its name: M3U8 leaves a comment where the location would be, XSPF, CSV and the backup carry the row without one, and PLS holds only real files — and the locations that are there are paths on this phone.</div>
         {:else if kind === "sleep"}
             <div class="sheet-title">Sleep timer</div>
             <div class="sheet-sec">Pause after</div>