Fix textarea padding in Safari

Use `box-sizing: border-box` to include padding in the width of the textarea.

Also,
> Box-sizing should be border-box by default
from [csswg](https://wiki.csswg.org/ideas/mistakes)

public/main.css

@@ -25,6 +25,10 @@
     --textarea-border-dark: #454545;
 }
 
+* {
+    box-sizing: border-box;
+}
+
 body {
     color: var(--text-light);
     background-color: var(--bg-light);