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/assets/styles/_fonts.scss

@@ -33,3 +33,10 @@
 $font-normal    : 'Monaco', 'Menlo', 'Ubuntu Mono', 'Consolas', 'source-code-pro', monospace;
 $font-artistic  : 'Yesteryear', cursive;
 $font-main      : 'Slabo 27px';
+
+$font-size-smaller  : 14px;
+$font-size-small    : 16px;
+$font-size-normal   : 18px;
+$font-size-large    : 22px;
+$font-size-larger   : 26px;
+$font-size-largest  : 40px;

src/presentation/assets/styles/_globals.scss

@@ -20,4 +20,5 @@ a {
 body {
     background: $color-background;
     font-family: $font-main;
+    font-size: $font-size-normal;
 }

src/presentation/assets/styles/_mixins.scss

@@ -1,4 +1,5 @@
 @use "@/presentation/assets/styles/colors" as *;
+@use "@/presentation/assets/styles/fonts" as *;
 
 @mixin hover-or-touch($selector-suffix: '', $selector-prefix: '&') {
   @media (hover: hover) {
@@ -92,6 +93,7 @@
 
 @mixin flat-button($disabled: false) {
   @include reset-button;
+  $font-size: $font-size-normal;
 
   @if $disabled {
     color: $color-primary-light;