release: v2.1.0

Version bumped in server.py, pyproject.toml, the PWA header and the
docs/api.md example; SW cache v122 -> v123.

docs/api.md

@@ -55,7 +55,7 @@ Returns server version for compatibility checking.
 
 **Example response:**
 ```json
-{"version": "2.0.0"}
+{"version": "2.1.0"}
 ```
 
 ### `GET /api/status`