Fix Playground link to use base64-encoded blueprint

Percent-encoded JSON caused 'Invalid blueprint' errors. Base64
encoding is the recommended format for blueprint URLs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dd32

README.md

@@ -2,7 +2,7 @@
 
 A WordPress plugin that allows multiple authors to edit a single post via shared invite links. Compatible with WordPress collaborative editing.
 
-[Test in WordPress Playground](https://playground.wordpress.net/#%7B%22landingPage%22%3A%22/wp-admin/post-new.php%22%2C%22login%22%3Atrue%2C%22steps%22%3A%5B%7B%22step%22%3A%22installPlugin%22%2C%22pluginData%22%3A%7B%22resource%22%3A%22git%3Adirectory%22%2C%22url%22%3A%22https%3A//github.com/dd32/multi-author-posts%22%2C%22ref%22%3A%22HEAD%22%2C%22refType%22%3A%22refname%22%7D%7D%5D%7D)
+[Test in WordPress Playground](https://playground.wordpress.net/#eyJsYW5kaW5nUGFnZSI6Ii93cC1hZG1pbi9wb3N0LW5ldy5waHAiLCJsb2dpbiI6dHJ1ZSwic3RlcHMiOlt7InN0ZXAiOiJpbnN0YWxsUGx1Z2luIiwicGx1Z2luRGF0YSI6eyJyZXNvdXJjZSI6ImdpdDpkaXJlY3RvcnkiLCJ1cmwiOiJodHRwczovL2dpdGh1Yi5jb20vZGQzMi9tdWx0aS1hdXRob3ItcG9zdHMiLCJyZWYiOiJIRUFEIiwicmVmVHlwZSI6InJlZm5hbWUifX1dfQ==)
 
 ## Features