Put the chain first in the sidebar

The source images took the top of the sidebar and most of its height,
which made the list of files look like the subject of the window rather
than its input. The chain is what the interface is about, so it now comes
first with the selected stage's params right below it, taking the free
space; the sources sit at the bottom in a collapsed expander whose label
carries the count, with their own "Add images" button instead of the one
that used to be in the header bar.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/gui/window.hpp

@@ -48,6 +48,7 @@ private:
     void closeLoadWindow();
     void cancelLoad();
     void appendSourceRow(const std::string& path);
+    void updateSourcesLabel();  // the collapsed expander shows the count
     void removeSource(Gtk::ListBoxRow* row);
 
     Gtk::Widget& buildChainSection();
@@ -139,6 +140,7 @@ private:
     std::unique_ptr<Gtk::Window> loadWindow_;
     Gtk::ProgressBar* loadBar_ = nullptr;
     Gtk::Label* loadFileLabel_ = nullptr;
+    Gtk::Expander sourcesExpander_;
     Gtk::ListBox sourceList_;
     Gtk::SpinButton widthSpin_;
     Gtk::SpinButton heightSpin_;