feat(ext/popup): rename sig-block--blue to --gold for accuracy

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

extension/src/popup/components/__tests__/fields.test.ts

@@ -67,9 +67,9 @@ describe('renderConcealedRow', () => {
 });
 
 describe('renderSignatureBlock', () => {
-  it('default accent is blue', () => {
+  it('default accent is gold', () => {
     const html = renderSignatureBlock({ children: '<p>hi</p>' });
-    expect(html).toContain('sig-block--blue');
+    expect(html).toContain('sig-block--gold');
     expect(html).toContain('<p>hi</p>');
   });