happy path working with stub companion app

companion/src/base64url.ts

@@ -0,0 +1,22 @@
+// base64url (RFC 4648 §5), no padding — interfaces.md's shared conventions.
+// Same implementation as extension/src/base64url.ts; duplicated rather than
+// shared across the two independent build targets.
+
+export function base64UrlDecode(s: string): Uint8Array {
+    const unpadded = s.replace(/-/g, '+').replace(/_/g, '/');
+    const padded = unpadded + '='.repeat((4 - (unpadded.length % 4)) % 4);
+    const binary = atob(padded);
+    const bytes = new Uint8Array(binary.length);
+    for (let i = 0; i < binary.length; i++) {
+        bytes[i] = binary.charCodeAt(i);
+    }
+    return bytes;
+}
+
+export function base64UrlEncode(bytes: Uint8Array): string {
+    let binary = '';
+    for (const byte of bytes) {
+        binary += String.fromCharCode(byte);
+    }
+    return btoa(binary).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
+}