Split the sidebar into Chain and Setup tabs, with a draggable chain list

The sidebar grew crowded as the chain got longer. The chain and the params
of the selected stage now share a Chain tab through a vertical split the
user drags, and the sources, canvas and seed move to a Setup tab. Stage
rows are compact, with their move and remove buttons shown only on the
hovered, selected or focused row.

Add effect had stopped opening: its menu of fifteen effects no longer fit
below the button, and GTK does not show a popover that does not fit. The
menu now scrolls past 480 pixels, and the button sits beside the chain
heading, where the split cannot push it down the screen.

The window code becomes an effector_gui library so a new GUI test binary
can drive the real window under Xvfb; ctest skips it when xvfb-run is
missing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/gui/window.hpp

@@ -29,6 +29,8 @@ public:
 private:
     void buildUi();
     Gtk::Widget& buildSidebar();
+    Gtk::Widget& buildChainTab();
+    Gtk::Widget& buildSetupTab();
     Gtk::Widget& buildPreview();
     void buildParamWidgets();
     ParamValues paramValues() const;
@@ -72,7 +74,7 @@ private:
     void closeLoadWindow();
     void cancelLoad();
     void appendSourceRow(const std::string& path);
-    void updateSourcesLabel();  // the collapsed expander shows the count
+    void updateSourcesLabel();  // the heading shows the count
     // The whole sources section only shows while the chain asks for sources.
     void updateSourcesVisible();
 
@@ -183,7 +185,8 @@ private:
     std::unique_ptr<Gtk::Window> loadWindow_;
     Gtk::ProgressBar* loadBar_ = nullptr;
     Gtk::Label* loadFileLabel_ = nullptr;
-    Gtk::Expander sourcesExpander_;
+    Gtk::Box sourcesSection_;  // hidden while the chain needs no sources
+    Gtk::Label sourcesHeading_;
     Gtk::ListBox sourceList_;
     Gtk::DropDown formatDrop_;
     Gtk::SpinButton widthSpin_;