Normalize and improve font sizes

This commit standardizes font sizes across components for a uniform
look. The icon sizes, font weights and line heights are also adjusted
accordingly for better standardization and simplicity.

- Introduce variables for standard font sizes, enhancing
  maintainability.
- Remove explicit pixel values, replaced with scalable units based on
  root size.
- Remove workaround for line-height adoptation of bigger font-size.
- Use consistent small font-size for the code area.
- Adjust checkbox tick to scale with font size.

src/presentation/components/Code/TheCodeArea.vue

@@ -203,6 +203,7 @@ function getDefaultCode(language: ScriptingLanguage): string {
     width: 100%;
     height: 100%;
     overflow: auto;
+    font-size: $font-size-small;
     &__highlight {
       background-color: $color-secondary-light;
       position: absolute;