Load source images in the background with a progress window

Decoding a batch of photographs took seconds on the main thread, so the
window froze with no sign of what was happening. loadImages now takes an
optional LoadProgress (per-file callback plus a cancel flag) and hands files
out from a shared cursor, so one large file cannot stall the queue behind it.

The GUI decodes off the main thread and shows a progress window with the
count, the current file and a Cancel button; it only appears if the batch is
still running after 250 ms. Headless mode keeps a "reading n/total" line on
stderr when it is attached to a terminal.

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

src/gui/window.hpp

@@ -2,6 +2,7 @@
 
 #include <gtkmm.h>
 
+#include <atomic>
 #include <cstdint>
 #include <memory>
 #include <mutex>
@@ -34,7 +35,12 @@ private:
     void onAddImages();
     void onImagesChosen(const Glib::RefPtr<Gio::AsyncResult>& result,
                         const Glib::RefPtr<Gtk::FileDialog>& dialog);
-    void addPaths(const std::vector<std::string>& paths);
+    void addPaths(const std::vector<std::string>& paths);  // decodes in the background
+    void onLoadProgress();    // dispatcher: one file finished decoding
+    void onLoadFinished();    // dispatcher: the batch is done or was cancelled
+    void showLoadWindow();
+    void closeLoadWindow();
+    void cancelLoad();
     void appendSourceRow(const std::string& path);
     void removeSource(Gtk::ListBoxRow* row);
 
@@ -65,6 +71,22 @@ private:
     std::uint64_t seed_ = 0;
     bool suspendRender_ = true;
 
+    // Source loading. Decoding a batch of large photographs takes seconds, so
+    // it runs off the main thread and reports into a small progress window.
+    std::thread loader_;
+    Glib::Dispatcher loadProgress_;
+    Glib::Dispatcher loadFinished_;
+    std::atomic<bool> loadCancel_{false};
+    std::atomic<std::size_t> loadDone_{0};
+    std::size_t loadTotal_ = 0;
+    std::mutex loadMutex_;
+    std::string loadCurrent_;
+    std::vector<Image> loadedImages_;
+    std::vector<LoadFailure> loadFailures_;
+    std::vector<std::string> loadQueue_;  // chosen while another batch was loading
+    bool loading_ = false;
+    sigc::connection loadWindowDelay_;
+
     // Render job
     Glib::Dispatcher renderDone_;
     std::thread worker_;
@@ -78,6 +100,11 @@ private:
 
     // Widgets
     Gtk::Label banner_;
+    Gtk::Button* addButton_ = nullptr;
+    // Built fresh with the progress window, and null whenever it is closed.
+    std::unique_ptr<Gtk::Window> loadWindow_;
+    Gtk::ProgressBar* loadBar_ = nullptr;
+    Gtk::Label* loadFileLabel_ = nullptr;
     Gtk::ListBox sourceList_;
     Gtk::SpinButton widthSpin_;
     Gtk::SpinButton heightSpin_;