feat: add image opacity, config migration, settings

Add image overlay opacity support and related UI/settings, plus robust config migration and error handling.

Key changes:
- UI: image overlay now uses QGraphicsOpacityEffect, hides when disabled, and updates opacity on state changes (src/interface/dialogs/image.py).
- New NotificationPopup dialog for user alerts (src/interface/dialogs/notificationpopup.py).
- Settings dialog: added Image Opacity control, adjusted labels, persist/rollback behavior on cancel, and include opacity when saving (src/interface/dialogs/settings.py).
- State: added properties for image_opacity and image_enabled with change notifications; renamed image_width setter param for clarity (src/utils/data/state.py).
- Config: switch from config.yml to config.ini, implement one-time migration, typed read/write of values, validation/error handling, automatic backups of corrupted files and user notification (src/utils/config/config.py).
- Settings save: accept and persist image_opacity (src/utils/config/settings.py).
- Startup: initialize QApplication earlier and pass app to run_gui (src/main.py).
- Minor: removed an extra blank line in log handler and small import cleanups.

These changes enable adjustable overlay transparency, improve config reliability, and surface errors to users while preserving/backuping old configs.

src/interface/dialogs/notificationpopup.py

@@ -0,0 +1,17 @@
+from PySide6.QtWidgets import QMessageBox
+import qdarktheme
+
+class NotificationPopup(QMessageBox):
+    def __init__(self, title, text, infotext=None, parent=None):
+        super().__init__(parent)
+
+        qdarktheme.setup_theme("auto")
+
+        self.setIcon(QMessageBox.Icon.Information)
+        self.setWindowTitle(title)
+        self.setText(text)
+        
+        if infotext is not None:
+            self.setInformativeText(infotext)
+            
+        self.setStandardButtons(QMessageBox.StandardButton.Ok)