Add bulk multi-photo upload to the desktop app

Mirror the Android bulk feature in the Qt uploader: choose several photos
at once, each a card with its own caption, date, and upload status, and
post the batch.

- Per-photo cards (PhotoItemWidget) in a scrollable list; retry re-posts
  only the failures.
- One upload per photo with a per-item UUID slug suffix so a batch posted
  in the same second cannot collide.
- Image prep runs off the UI thread via QtConcurrent so the window stays
  responsive.
- A "Simultaneous uploads" setting (1–8, default 3) caps how many photos
  are prepared/uploaded at once; the rest queue and fill freed slots.

desktop/photoitemwidget.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <QString>
+#include <QWidget>
+
+class QDateEdit;
+class QLabel;
+class QLineEdit;
+class QPushButton;
+
+// One photo in the batch: thumbnail, caption, date, and its own upload status.
+// A single-photo post is just a batch of one, so the window runs one code path.
+class PhotoItemWidget : public QWidget
+{
+    Q_OBJECT
+public:
+    enum class Status { Pending, Preparing, Uploading, Success, Error };
+
+    explicit PhotoItemWidget(QString sourcePath, QWidget *parent = nullptr);
+
+    QString sourcePath() const { return m_sourcePath; }
+    QString caption() const;
+    QString date() const; // yyyy-MM-dd
+    Status status() const { return m_status; }
+
+    void setStatus(Status status, const QString &message = {}, const QString &url = {});
+
+signals:
+    void removeRequested(PhotoItemWidget *self);
+
+private:
+    QString m_sourcePath;
+    Status m_status = Status::Pending;
+
+    QLabel *m_thumb = nullptr;
+    QLineEdit *m_caption = nullptr;
+    QDateEdit *m_date = nullptr;
+    QLabel *m_statusLabel = nullptr;
+    QPushButton *m_remove = nullptr;
+};