input.css: set weight to 600 for bold

Prevents the ugly browser enforced bold that we had all this while.

input.css

···
     @font-face {
         font-family: "InterVariable";
         src: url("/static/fonts/InterVariable-Italic.woff2") format("woff2");
-        font-weight: normal;
+        font-weight: 400;
         font-style: italic;
+        font-display: swap;
+    }
+
+    @font-face {
+        font-family: "InterVariable";
+        src: url("/static/fonts/InterVariable.woff2") format("woff2");
+        font-weight: 600;
+        font-style: normal;
         font-display: swap;
     }