Redirect after saving user CSS/JS so changes apply immediately (#9006)

Saving custom CSS/JS rendered the response from the pre-save state (no
Post/Redirect/Get), so the stylesheet cache-busting URL was stale and
the change only appeared after a manual reload, which reads as erratic
behavior. Redirect back to the extension config after save, matching
the pattern used elsewhere in the codebase.

Fixes #8795

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/core-extensions/UserCSS/metadata.json

@@ -2,7 +2,7 @@
 	"name": "User CSS",
 	"author": "hkcomori, Marien Fressinaud",
 	"description": "Give possibility to overwrite the CSS with a user-specific rules.",
-	"version": "1.1.1",
+	"version": "1.1.2",
 	"entrypoint": "UserCSS",
 	"type": "user"
 }