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/mainwindow.h

@@ -0,0 +1,53 @@
+#pragma once
+
+#include <QMainWindow>
+
+#include "settings.h"
+#include "uploadclient.h"
+
+class QDateEdit;
+class QLabel;
+class QLineEdit;
+class QPushButton;
+class QStackedWidget;
+
+class MainWindow : public QMainWindow
+{
+    Q_OBJECT
+public:
+    explicit MainWindow(QWidget *parent = nullptr);
+
+private:
+    QWidget *buildSettingsPage();
+    QWidget *buildPostPage();
+
+    void showSettings();
+    void showPost();
+
+    void saveAndTest();
+    void chooseImage();
+    void submit();
+
+    Settings m_settings;
+
+    QStackedWidget *m_stack = nullptr;
+
+    // Settings page
+    QLineEdit *m_baseUrl = nullptr;
+    QLineEdit *m_username = nullptr;
+    QLineEdit *m_password = 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;
+    QPushButton *m_postButton = nullptr;
+    QLabel *m_postStatus = nullptr;
+    QLabel *m_lastPost = nullptr;
+
+    QString m_imagePath; // currently selected source image
+};