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/uploadclient.cpp

@@ -26,7 +26,7 @@ QByteArray UploadClient::authHeader() const
 }
 
 void UploadClient::upload(const QString &filePath, const QString &fileName,
-                          const QString &caption, const QString &date)
+                          const QString &caption, const QString &date, const QString &tags)
 {
     auto *file = new QFile(filePath);
     if (!file->open(QIODevice::ReadOnly)) {
@@ -57,6 +57,7 @@ void UploadClient::upload(const QString &filePath, const QString &fileName,
     };
     multiPart->append(textPart(QStringLiteral("caption"), caption));
     multiPart->append(textPart(QStringLiteral("date"), date));
+    multiPart->append(textPart(QStringLiteral("tags"), tags));
 
     QNetworkRequest request(QUrl(m_base + QStringLiteral("/upload")));
     request.setRawHeader("Authorization", authHeader());