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.

android/app/src/main/java/ws/inflo/photogallery/MainActivity.kt

@@ -81,6 +81,7 @@ class MainActivity : ComponentActivity() {
                             )
                         },
                         onCaptionChange = viewModel::setCaption,
+                        onTagsChange = viewModel::setTags,
                         onDateChange = viewModel::setDate,
                         onRemove = viewModel::removeItem,
                         onSubmit = viewModel::submit,