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/imageprep.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <QByteArray>
+#include <QString>
+
+// Turns a picked image into a file the gallery's upload API accepts.
+//
+// Three things must be true of the result: no EXIF (the gallery publishes the
+// uploaded file verbatim as its full-size download, so embedded GPS would be
+// public), a .jpg or .png extension (server.go rejects anything else, HEIC
+// included), and under nginx's 20M body cap.
+//
+// JPEG and PNG take a lossless route — whole metadata segments are dropped
+// without touching the compressed image data — so the published file keeps the
+// original sensor pixels. Anything else, or a file whose EXIF orientation must
+// be baked in, is decoded and re-encoded as JPEG (HEIC via Qt's qtimageformats
+// plugin). The strippers are a direct port of the Android app's.
+class ImagePrep
+{
+public:
+    struct Prepared {
+        QString filePath;
+        QString note; // non-empty when the app departed from a byte-exact copy
+        QString error;
+        bool ok() const { return error.isEmpty(); }
+    };
+
+    // Writes the upload-ready file into cacheDir as baseName.(jpg|png).
+    static Prepared prepare(const QString &sourcePath, const QString &cacheDir,
+                            const QString &baseName);
+
+private:
+    static QByteArray stripJpegSegments(const QByteArray &src); // empty on parse failure
+    static QByteArray stripPngChunks(const QByteArray &src);
+    static bool isJpeg(const QByteArray &b);
+    static bool isPng(const QByteArray &b);
+};