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/TheFooter/TheFooter.vue

@@ -124,7 +124,6 @@ export default defineComponent({
       }
     }
     flex-wrap: wrap;
-    font-size: 1rem;
     font-family: $font-normal;
     &__item:not(:first-child) {
       &::before {