feat(preview): add sandboxed live code previews

Render validated HTML, SVG, JSX, and TSX fences locally while preserving tool context and preventing explanatory JSON from triggering actions.

Co-authored-by: Cursor <cursoragent@cursor.com>

bin/check.sh

@@ -25,6 +25,16 @@ else
     echo "-- skipped: interface/web/node_modules missing (npm install)"
 fi
 
+echo "== frontend unit tests =="
+# The JSX/TSX transform behind the preview window is a pure module with a
+# node --test suite. Nothing else in the frontend has tests, so this is cheap;
+# without it the transform's silent-wrong cases go unguarded.
+if [ -d interface/web/node_modules ]; then
+    (cd interface/web && npm test) || fail=1
+else
+    echo "-- skipped: interface/web/node_modules missing (npm install)"
+fi
+
 echo "== powershell parse =="
 # The Windows installer has died at parse twice. Cheap to catch here if pwsh
 # happens to be installed on the Linux box; the ASCII guard in tests/ is the