fix(links): use plus icon for logo upload placeholder

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/widget-system/links-page-contract.test.cjs

@@ -22,6 +22,7 @@ test('native links are the only friend-link store', () => {
   assert.match(template, /jio_page_links_html\(/)
   assert.match(template, /jio_submit_links_modal\(/)
   assert.match(helper, /get_bookmarks\(/)
+  assert.match(helper, /assets\/icons\/upload-add\.svg/)
 })
 
 test('the retired friend-link option and admin REST surface stay gone', () => {