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/mainwindow.h

@@ -1,15 +1,18 @@
 #pragma once
 
+#include <QList>
 #include <QMainWindow>
 
 #include "settings.h"
-#include "uploadclient.h"
 
 class QDateEdit;
 class QLabel;
 class QLineEdit;
 class QPushButton;
+class QSpinBox;
 class QStackedWidget;
+class QVBoxLayout;
+class PhotoItemWidget;
 
 class MainWindow : public QMainWindow
 {
@@ -25,8 +28,15 @@ private:
     void showPost();
 
     void saveAndTest();
-    void chooseImage();
+    void chooseImages();
+    void addItem(const QString &path);
+    void removeItem(PhotoItemWidget *item);
+    void clearItems();
     void submit();
+    void pumpQueue();                    // starts queued items up to the concurrency cap
+    void startItem(PhotoItemWidget *item);
+    int inFlightCount() const;           // items currently preparing or uploading
+    void refreshPosting();               // updates the Post button + summary from item states
 
     Settings m_settings;
 
@@ -36,18 +46,24 @@ private:
     QLineEdit *m_baseUrl = nullptr;
     QLineEdit *m_username = nullptr;
     QLineEdit *m_password = nullptr;
+    QSpinBox *m_concurrency = nullptr;
     QPushButton *m_saveButton = nullptr;
     QPushButton *m_settingsBack = nullptr;
     QLabel *m_settingsStatus = nullptr;
 
     // Post page
-    QLabel *m_preview = nullptr;
-    QLineEdit *m_caption = nullptr;
-    QDateEdit *m_date = nullptr;
-    QPushButton *m_chooseButton = nullptr;
+    QVBoxLayout *m_itemsLayout = nullptr; // holds the PhotoItemWidgets
+    QList<PhotoItemWidget *> m_items;
+    QLabel *m_emptyHint = nullptr;
     QPushButton *m_postButton = nullptr;
-    QLabel *m_postStatus = nullptr;
+    QLabel *m_postSummary = nullptr;
     QLabel *m_lastPost = nullptr;
 
-    QString m_imagePath; // currently selected source image
+    // Bulk-post state: items waiting for a free upload slot, plus the batch's
+    // credentials captured once at submit time.
+    QList<PhotoItemWidget *> m_queue;
+    QString m_batchBaseUrl;
+    QString m_batchUsername;
+    QString m_batchPassword;
+    QString m_batchCacheDir;
 };