feat: add product concept subtitle to radars

A Radar gains an optional Concept — a short subtitle naming the product
and its typical performances, rendered under the title and wrapping to a
balanced second line when long. Old radars default to an empty Concept
on load.

.oxfmtrc.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "./node_modules/oxfmt/configuration_schema.json",
+  "semi": false,
+  "singleQuote": false,
+  "ignorePatterns": []
+}