refactor: cleanup backend structure

frontend/src/util/codecs.ts

@@ -1,11 +1,11 @@
 // https://developer.mozilla.org/en-US/docs/Glossary/Base64#the_unicode_problem.
-export function fromBase64(string: String) {
+export function fromBase64(string) {
   const binString = atob(string);
   const bytes = Uint8Array.from(binString, (m) => m.codePointAt(0))
   return new TextDecoder().decode(bytes);
 }
 
-export function toBase64(string: String) {
+export function toBase64(string) {
   const bytes = new TextEncoder().encode(string)
   const binString = String.fromCodePoint(...bytes);
   return btoa(binString);