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/flake.nix

@@ -0,0 +1,38 @@
+{
+  description = "photogallery uploader — Qt6 C++ desktop app";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+
+        qtPlugins = pkgs.lib.makeSearchPathOutput "out" "lib/qt-6/plugins" [
+          pkgs.qt6.qtbase
+          pkgs.qt6.qtwayland
+          pkgs.qt6.qtimageformats
+        ];
+      in {
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs; [
+            cmake
+            ninja
+            pkg-config
+            qt6.qtbase
+            qt6.qtwayland       # native Wayland QPA plugin
+            qt6.qtimageformats  # HEIF/HEIC + others, read by QImage
+            qt6Packages.qtkeychain # password → libsecret / kwallet
+          ];
+
+          shellHook = ''
+            # Let a binary run straight from the build dir find Qt's platform
+            # (xcb + wayland) and image-format plugins without wrapQtAppsHook.
+            export QT_PLUGIN_PATH="${qtPlugins}''${QT_PLUGIN_PATH:+:$QT_PLUGIN_PATH}"
+          '';
+        };
+      });
+}