docs: archival readme notice

unfortunate.

README.md

@@ -1,3 +1,8 @@
+> [!IMPORTANT]
+> As of **6th April 2026**, this repository is archived.<br>
+> The website will be continued to be updated privately, the current state of the source code remains here due to the license.<br>
+> You can still help out! Contact me to see the private codebase.
+
 <p align="center">
   <img src="public/logo.svg" alt="TomodachiShare Logo" width="128" />
 </p>