Move ai-stack and paintplus vendored source under vendor/

Matches the existing vendor/easy-asterisk convention (used by
services/asterisk.sh) instead of two one-off top-level directories that
cluttered the repo root and didn't look like anything else next to
setup.sh, lib/, services/, extras/. Only the two services' own SRC_DIR
path resolution and header comments needed updating — nothing else in
the repo referenced the old ./ai-stack / ./paintplus paths.

Also documents vendor/ in README.md's Layout section.

vendor/paintplus/frontend/.babelrc

@@ -0,0 +1,8 @@
+{
+	"presets": ["@babel/preset-env"],
+	"plugins": [
+		["@babel/plugin-transform-runtime", {
+			"regenerator": true
+		}]
+	]
+}