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/photoitemwidget.h

@@ -21,6 +21,7 @@ public:
     QString sourcePath() const { return m_sourcePath; }
     QString caption() const;
     QString date() const; // yyyy-MM-dd
+    QString tags() const; // raw comma-separated; the server normalizes
     Status status() const { return m_status; }
 
     void setStatus(Status status, const QString &message = {}, const QString &url = {});
@@ -35,6 +36,7 @@ private:
     QLabel *m_thumb = nullptr;
     QLineEdit *m_caption = nullptr;
     QDateEdit *m_date = nullptr;
+    QLineEdit *m_tags = nullptr;
     QLabel *m_statusLabel = nullptr;
     QPushButton *m_remove = nullptr;
 };