Add GTK theme integration for Linux desktop environments

- Add _setup_linux_theming() called before QApplication creation
- Auto-detect libqgtk3.so plugin from common Qt6 plugin paths
- Sets QT_QPA_PLATFORMTHEME=gtk3 so Qt follows system GTK theme
- Falls back to qt6ct if gtk3 plugin not found
- Falls back to Fusion style if no platform theme available
- Respects user-set QT_QPA_PLATFORMTHEME (never overrides)

src/main.py

@@ -1043,8 +1043,53 @@ class MainWindow(QMainWindow):
         event.accept()
 
 
+# GTK3 platform theme paths for Linux desktop integration
+_GTK3_PLUGIN_PATHS = [
+    "/usr/lib/qt6/plugins/platformthemes/libqgtk3.so",
+    "/usr/lib/x86_64-linux-gnu/qt6/plugins/platformthemes/libqgtk3.so",
+    "/usr/lib64/qt6/plugins/platformthemes/libqgtk3.so",
+    "/usr/lib/aarch64-linux-gnu/qt6/plugins/platformthemes/libqgtk3.so",
+]
+
+
+def _setup_linux_theming():
+    """Configure Qt to follow the system GTK theme on Linux.
+    
+    Must be called BEFORE QApplication is created.
+    Respects user-set QT_QPA_PLATFORMTHEME (doesn't override it).
+    Falls back through available platform themes: gtk3 → qt6ct → none.
+    """
+    if sys.platform != "linux":
+        return
+
+    if "QT_QPA_PLATFORMTHEME" in os.environ:
+        return  # User already set it
+
+    # Check for libqgtk3.so (official Qt GTK integration)
+    for path in _GTK3_PLUGIN_PATHS:
+        if os.path.exists(path):
+            os.environ["QT_QPA_PLATFORMTHEME"] = "gtk3"
+            logger.debug(f"Using GTK3 platform theme: {path}")
+            return
+
+    # Check for qt6ct (Qt configuration tool)
+    if os.path.exists("/usr/bin/qt6ct"):
+        os.environ["QT_QPA_PLATFORMTHEME"] = "qt6ct"
+        logger.debug("Using qt6ct platform theme")
+        return
+
+    logger.debug("No GTK platform theme plugin found, using Qt default style")
+
+
 if __name__ == "__main__":
+    _setup_linux_theming()
+
     app = QApplication(sys.argv)
+
+    # Apply Fusion as fallback style if no platform theme was applied
+    if not os.environ.get("QT_QPA_PLATFORMTHEME"):
+        app.setStyle("Fusion")
+
     window = MainWindow()
     window.show()
     sys.exit(app.exec())