Add a tags field to the Android and desktop uploaders

Each photo card gains a free-text tags input, sent as a raw
comma-separated `tags` form field on upload. The server normalizes it
(parseTags), so the clients stay dumb — no tag parsing on-device.

- Android: PickedItem.tags + setTags; threaded through UploadQueue →
  UploadWorker → UploadClient's multipart.
- Desktop: a tags QLineEdit per PhotoItemWidget; UploadClient.upload sends
  it.

desktop/mainwindow.cpp

@@ -313,7 +313,7 @@ void MainWindow::startItem(PhotoItemWidget *item)
                             refreshPosting();
                         });
                 client->upload(prepared.filePath, QFileInfo(prepared.filePath).fileName(),
-                               caption, item->date());
+                               caption, item->date(), item->tags());
             });
     watcher->setFuture(QtConcurrent::run([source, cacheDir, slug]() {
         return ImagePrep::prepare(source, cacheDir, slug);