Refactor toast notifications and modal dialog implementation

- Updated SCSS for toast notifications to support multiple toast types (success, info, warning, error) and improved layout.
- Added new SVG icons for error, info, success, and warning notifications.
- Created separate HTML templates for toast notifications and modal dialogs.
- Enhanced the dev panel with buttons to test different toast notifications and modal dialogs.

backend/templates/modal_dialog.html

@@ -0,0 +1,6 @@
+{# Needed by the frontend modal_dialog module. #}
+<dialog id="modal-dialog">
+    <div class="content"></div>
+    <input type="button" class="ok" value="OK">
+    <input type="button" class="cancel" value="Cancel">
+</dialog>