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/CMakeLists.txt

@@ -0,0 +1,23 @@
+cmake_minimum_required(VERSION 3.20)
+project(photogallery_uploader LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_AUTOMOC ON)
+
+find_package(Qt6 REQUIRED COMPONENTS Widgets Network)
+find_package(Qt6Keychain REQUIRED)
+
+add_executable(uploader
+    main.cpp
+    mainwindow.cpp
+    settings.cpp
+    uploadclient.cpp
+    imageprep.cpp
+)
+
+target_link_libraries(uploader PRIVATE
+    Qt6::Widgets
+    Qt6::Network
+    Qt6Keychain::Qt6Keychain
+)