fix(siril): expose adwaita-qt6 style plugin for dark theme

modules/nixos/astro.nix

@@ -16,9 +16,18 @@ let
     # isn't on the runtime PATH), so without the --prefix PATH below the GUI
     # fails with "Python not found in system PATH" and never creates
     # ~/.local/share/siril/venv. We add the same interpreter siril references.
+    #
+    # The VeraLux PyQt6 wheel ships only the built-in Windows/Fusion styles, so
+    # the global QT_STYLE_OVERRIDE=adwaita-dark (inherited from the session) is
+    # rejected ("invalid style override") and windows fall back to a light
+    # theme. We expose nixpkgs adwaita-qt6's style plugin on QT_PLUGIN_PATH so
+    # the override resolves. This works because nixpkgs qt6 and the bundled Qt
+    # share a major.minor (6.11); if they ever drift the plugin is simply
+    # ignored again (harmless warning + Fusion fallback, no loss of function).
     postBuild = ''
       wrapProgram $out/bin/siril \
         --prefix PATH : ${lib.makeBinPath [ pkgs.python3 ]} \
+        --prefix QT_PLUGIN_PATH : ${pkgs.adwaita-qt6}/lib/qt-6/plugins \
         --prefix LD_LIBRARY_PATH : ${lib.makeLibraryPath (with pkgs; [
           stdenv.cc.cc.lib   # libstdc++ / libgcc_s / libgomp  (numpy etc.)
           zlib