fix: remove dead DEBUG import from helpers.js

The DEBUG constant from shared/constants was imported but never used.
updateDebugBanner() uses isDebug() from shared/log instead.

Closes #144

src/popup/views/helpers.js

@@ -1,6 +1,5 @@
 // Shared DOM helpers used by all views.
 
-const { DEBUG } = require("../../shared/constants");
 const { isDebug } = require("../../shared/log");
 const {
     formatUsd,