Add Qt6 desktop uploader

A native Qt6/C++ desktop client for the gallery's POST /upload API, in
desktop/ with its own flake (kept separate from the Go server module the
same way android/ is). Single-photo parity with the Android app:

- multipart upload with preemptive HTTP Basic auth and retry-aware outcome
  classification (uploadclient)
- JPEG/PNG metadata stripped byte-for-byte to drop embedded GPS, EXIF
  orientation baked in, and a quality/scale valve for the 20M nginx cap;
  HEIC decodes via the qtimageformats plugin (imageprep)
- base URL / username in QSettings, password in the system keychain via
  qtkeychain; https required and a trailing /upload stripped (settings)
- settings and post screens in a QStackedWidget (mainwindow)

Native Wayland or xcb is selected at runtime. Bulk upload, EXIF
capture-date prefill, and moving image prep off the UI thread are left as
follow-ups.

desktop/main.cpp

@@ -0,0 +1,15 @@
+#include <QApplication>
+
+#include "mainwindow.h"
+
+int main(int argc, char **argv)
+{
+    QApplication app(argc, argv);
+    app.setApplicationName(QStringLiteral("photogallery-uploader"));
+    app.setOrganizationName(QStringLiteral("inflo.ws"));
+
+    MainWindow window;
+    window.resize(440, 680);
+    window.show();
+    return app.exec();
+}