frontend: set default font-size

The last commits introduced the '62.5% Font Size Trick' which
almost worked perfectly, except for components that do not
set font-size at all and inherit from the base font-size of
the parent.

These components now render with small 10px font-size instead
of 16px.

Fixed by changing the app style (most outer parent class) to
1.6rem which equals to 16px, so all components that do not
define font-size inherit 16px.

Author: thisconnect

frontends/web/src/style/layout.css

@@ -56,6 +56,7 @@
 .app {
     display: flex;
     flex: 1 1 auto;
+    font-size: var(--size-root);
     min-height: 100vh;
     /* mobile viewport bug fix */
     min-height: -webkit-fill-available;

frontends/web/src/style/variables.css

@@ -48,6 +48,7 @@
     --color-error-border: rgba(227, 6, 19, .33);
 
     /* font sizes */
+    --size-root: 1.6rem; /* 16px base for body */
     --size-extra-large: 3.5rem;
     --size-large: 3.2rem;
     --size-large-mobile: 2.2rem;