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/toast.html

@@ -0,0 +1,8 @@
+{# Needed by the frontend toast module. #}
+<div id="toasts">
+    <div class="toast">
+        <img class="icon" width="24" height="24">
+        <div class="content"></div>
+        <span class="close button">✖</span>
+    </div>
+</div>