feat: extend MainWindow with QThread worker, settings panel, and batch dialog

- ConversionWorker runs pipeline on background QThread so the GUI stays
  responsive during conversion; emits file_done, preview_ready, finished
- MainWindow adds: output-format QComboBox, film-type QComboBox, Batch button
  that opens an AppConfig INI file and discovers files from batch.input_dir
- main.cpp: --batch and --config flags trigger CLI mode without Qt

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.cpp

@@ -1,35 +1,49 @@
 #include "cli/CliRunner.h"
-#include "gui/MainWindow.h"
-
-#include <QApplication>
 
 #include <algorithm>
 #include <iostream>
 #include <string>
 
+#ifndef NO_GUI
+#include "gui/MainWindow.h"
+#include <QApplication>
+#endif
+
 /**
  * @brief Application entry point.
  *
- * Supports two modes:
- * - GUI mode (default): launches the Qt MainWindow
- * - CLI mode (--cli flag): batch processes files without GUI
+ * Supports two operating modes:
+ *
+ * **GUI mode** (default when compiled with Qt):
+ *   Launches the Qt MainWindow.
+ *
+ * **CLI / batch mode** (activated by `--cli` or `--batch`):
+ *   Processes files from the command line or a config file without any GUI.
+ *   Progress is written to stderr; errors are logged but do not abort the batch.
+ *
+ * @note The `--batch` flag (or `--config <file>`) implies CLI mode.
  */
 int main(int argc, char* argv[]) {
-    // Check if CLI mode is requested
+    // Determine whether CLI/batch mode was requested.
     bool cli_mode = false;
     for (int i = 1; i < argc; ++i) {
-        if (std::string{argv[i]} == "--cli") {
+        const std::string arg{argv[i]};
+        if (arg == "--cli" || arg == "--batch" || arg == "--config") {
             cli_mode = true;
             break;
         }
     }
 
     if (cli_mode) {
-        // CLI batch mode (no Qt dependency)
+        // ── CLI / Batch mode (no Qt dependency) ─────────────────────────────
         auto config_result = photoconv::CliRunner::parse_args(argc, argv);
         if (!config_result.has_value()) {
-            std::cerr << config_result.error().format() << std::endl;
-            return 1;
+            // "Help requested" is not an error – exit 0.
+            const bool is_help = config_result.error().message == "Help requested";
+            if (!is_help) {
+                std::cerr << config_result.error().format() << std::endl;
+            }
+            return is_help ? 0 : 1;
         }
 
         photoconv::CliRunner runner;
@@ -39,16 +53,24 @@ int main(int argc, char* argv[]) {
             return 1;
         }
 
+        // Exit code 0 if at least one file was converted, 1 otherwise.
         return result.value() > 0 ? 0 : 1;
     }
 
-    // GUI mode
-    QApplication app(argc, argv);
+#ifndef NO_GUI
+    // ── GUI mode ─────────────────────────────────────────────────────────────
+    QApplication app{argc, argv};
     app.setApplicationName("Photo Converter");
     app.setApplicationVersion("0.1.0");
+    app.setOrganizationName("photo-converter");
 
     photoconv::MainWindow window;
     window.show();
 
     return app.exec();
+#else
+    std::cerr << "This build was compiled without GUI support.\n"
+                 "Use --cli or --batch mode.\n";
+    return 1;
+#endif
 }