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.cpp

@@ -47,6 +47,10 @@ PhotoItemWidget::PhotoItemWidget(QString sourcePath, QWidget *parent)
     m_date->setDisplayFormat(QStringLiteral("yyyy-MM-dd"));
     fields->addWidget(m_date);
 
+    m_tags = new QLineEdit;
+    m_tags->setPlaceholderText(QStringLiteral("tags (comma-separated)"));
+    fields->addWidget(m_tags);
+
     m_statusLabel = new QLabel;
     m_statusLabel->setOpenExternalLinks(true);
     m_statusLabel->setWordWrap(true);
@@ -70,6 +74,11 @@ QString PhotoItemWidget::date() const
     return m_date->date().toString(QStringLiteral("yyyy-MM-dd"));
 }
 
+QString PhotoItemWidget::tags() const
+{
+    return m_tags->text();
+}
+
 void PhotoItemWidget::setStatus(Status status, const QString &message, const QString &url)
 {
     m_status = status;
@@ -79,6 +88,7 @@ void PhotoItemWidget::setStatus(Status status, const QString &message, const QSt
     const bool editable = status == Status::Pending || status == Status::Error;
     m_caption->setEnabled(editable);
     m_date->setEnabled(editable);
+    m_tags->setEnabled(editable);
     m_remove->setEnabled(status != Status::Preparing && status != Status::Uploading);
 
     switch (status) {