Fix base64 encoding for browser environment

The bufferToBase64 function from @binect/js expects Node.js Buffer
objects but was receiving browser ArrayBuffer, causing "[object ArrayBuffer]"
to be sent instead of valid base64. Use browser-native btoa() instead.

Also updates tests to work with @binect/js integration.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -2,6 +2,9 @@
   "name": "binect-chrome",
   "version": "1.0.0",
   "description": "Chrome extension to send PDFs from cloud applications directly to Binect for physical mail delivery",
+  "dependencies": {
+    "@binect/js": "file:../binect-js"
+  },
   "scripts": {
     "build": "webpack --mode production",
     "dev": "webpack --mode development --watch",