UI polish: overlay color, debounce, and tooltips

- Added overlay color storage to settings import/export
- Removed generic keyboard shortcuts from menus and replaced them with localized tooltips
- Added QTimer debouncing to log history dialog to prevent status string spam
- Ensured export worker gracefully handles KeyErrors with standard return tuple

tests/test_image_processor.py

@@ -155,3 +155,20 @@ def test_calculate_grouping_score():
     # 1 center pixel = 80/80 = 1.0. 
     # Overall it should be a healthy percentage.
     assert res_9x9 > 10.0 # significant grouping
+
+def test_export_worker_error():
+    from app.qt.image_processor import _export_worker
+    
+    # 1. Provide a missing file to trigger an exception during Image.open()
+    res1 = _export_worker(("missing_file.png", {
+        "hue_min": 0, "hue_max": 360, "sat_min": 0, "sat_max": 100,
+        "val_min": 0, "val_max": 100, "exclude_bg": False,
+        "exclude_bg_rgb": (0, 0, 0), "exclude_bg_tolerance": 5,
+        "prefer_dark": False, "exclude_shapes": [], "exclude_ref_size": None,
+        "weights": {}
+    }))
+    assert res1 == ("missing_file.png", None, None, None, None, None)
+    
+    # 2. Provide an empty params dict to trigger KeyError before opening image
+    res2 = _export_worker(("dummy.png", {}))
+    assert res2 == ("dummy.png", None, None, None, None, None)