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/settings.cpp

@@ -3,6 +3,8 @@
 #include <QEventLoop>
 #include <QUrl>
 
+#include <algorithm>
+
 #include <qt6keychain/keychain.h>
 
 namespace {
@@ -35,6 +37,17 @@ void Settings::setUsername(const QString &value)
     m_settings.setValue(QStringLiteral("username"), value.trimmed());
 }
 
+int Settings::concurrency() const
+{
+    const int v = m_settings.value(QStringLiteral("concurrency"), kDefaultConcurrency).toInt();
+    return std::clamp(v, 1, kMaxConcurrency);
+}
+
+void Settings::setConcurrency(int value)
+{
+    m_settings.setValue(QStringLiteral("concurrency"), std::clamp(value, 1, kMaxConcurrency));
+}
+
 QString Settings::readPassword() const
 {
     QKeychain::ReadPasswordJob job(kService);