android: a folder of audio files, and a paste of track links

The paste door now reads itself the way the desktop's does: one track link
in the paste and every line is looked up on its own public page
(playlist_import_track_links), which is the way past Spotify's 100-track
public ceiling — the wait screen counts them as they arrive. A fourth door
imports a folder of audio files off this phone, picked with the storage
plugin's own document-tree picker; playlist_import_folder grew a path
argument for it, because on Android it had no dialog to open and always
answered nothing. A YouTube playlist link needed no door at all — the link
one already dispatches it.

src/mobile/HalfSheet.svelte

@@ -13,7 +13,7 @@
     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, coverFromPhoto, coverClear } from "./playlists.svelte.js";
-    import { DOORS, start as startImport } from "./importer.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, albumsBy, tracksOf, loadLibrary, refreshPlaylists, refreshRecents } from "./lib.svelte.js";
     import { lyr, nudge, searchCandidates, closeCandidates, chooseCandidate, forgetLyrics, pasteLyrics, setAutoSync, openTapSync, tapProgress, reduceVocals, backToOriginal } from "./lyrics.svelte.js";
@@ -146,6 +146,17 @@
             onsubmit: (v) => { push("import", {}); void startImport(door, v); },
         });
     }
+    /* the folder door picks FIRST: the review only opens for a folder that
+       was chosen, and a tree with no path on this phone cannot be walked */
+    async function folderDoor() {
+        closeHalf();
+        try {
+            const path = await pickImportFolder();
+            if (!path) return;
+            push("import", {});
+            void startFolder(path);
+        } catch (e) { oops(e); }
+    }
     /* find a local match — the desktop's repair panel, on a sheet: the
        library searched by the row's name, a pick relinks the row (an
        unknown placeholder adopts the track's identity; the original line
@@ -494,10 +505,10 @@
             <div class="sheet-title">Import a playlist</div>
             <div class="sheet-rows">
                 {#each DOORS as [door, icon, t, m] (door)}
-                    {@render row(icon, t, m, () => importPrompt(door))}
+                    {@render row(icon, t, m, () => (door === "folder" ? folderDoor() : importPrompt(door)))}
                 {/each}
             </div>
-            <div class="sec-note" style="padding-bottom: 14px">A playlist file or a folder of audio files imports on the desktop, where the files are.</div>
+            <div class="sec-note" style="padding-bottom: 14px">A playlist file — M3U, XSPF, PLS or CSV — imports on the desktop; the phone has no file picker for one yet.</div>
         {:else if kind === "plmatch" && shown?.arg?.item}
             {@const p = shown.arg.playlist}
             {@const item = shown.arg.item}