feat(headerspec): add UTF-8 encoding/decoding support

* Implement round-trip encoding/decoding for UTF-8 values in header functions.
* Update encodeHeaderValue and decodeHeaderValue to use base64 utility functions.
* Add tests for UTF-8 header value handling.
* Update Vite config to externalize base64 utility.

resolvespec-js/src/__tests__/headerspec.test.ts

@@ -126,11 +126,22 @@ describe('encodeHeaderValue / decodeHeaderValue', () => {
         expect(decoded).toBe(original);
     });
 
+    it('should round-trip UTF-8 values', () => {
+        const original = 'café ☕ 你好';
+        expect(decodeHeaderValue(encodeHeaderValue(original))).toBe(original);
+    });
+
     it('should decode __ prefixed values', () => {
         const encoded = '__' + btoa('hello');
         expect(decodeHeaderValue(encoded)).toBe('hello');
     });
 
+    it('should decode UTF-8 values with the __ prefix', () => {
+        const bytes = new TextEncoder().encode('café ☕');
+        const binary = Array.from(bytes, (byte) => String.fromCharCode(byte)).join('');
+        expect(decodeHeaderValue('__' + btoa(binary))).toBe('café ☕');
+    });
+
     it('should return plain values as-is', () => {
         expect(decodeHeaderValue('plain')).toBe('plain');
     });

resolvespec-js/src/headerspec/client.ts

@@ -1,4 +1,5 @@
 import { clientCacheKey, clientHeaders, mergeHeaders } from '../common/http';
+import { b64DecodeUnicode, b64EncodeUnicode } from '@warkypublic/artemis-kit/base64';
 import type {
   APIResponse,
   ClientConfig,
@@ -13,10 +14,7 @@ import type {
  * Encode a value with base64 and ZIP_ prefix for complex header values.
  */
 export function encodeHeaderValue(value: string): string {
-  if (typeof btoa === "function") {
-    return "ZIP_" + btoa(value);
-  }
-  return "ZIP_" + Buffer.from(value, "utf-8").toString("base64");
+  return "ZIP_" + b64EncodeUnicode(value);
 }
 
 /**
@@ -42,10 +40,7 @@ export function decodeHeaderValue(value: string): string {
 }
 
 function decodeBase64(str: string): string {
-  if (typeof atob === "function") {
-    return atob(str);
-  }
-  return Buffer.from(str, "base64").toString("utf-8");
+  return b64DecodeUnicode(str);
 }
 
 /**