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/.gitignore

@@ -0,0 +1,17 @@
+# OS generated files #
+.DS_Store
+.DS_Store?
+._*
+.Trashes
+ehthumbs.db
+Thumbs.db
+nbproject/
+.idea/
+.git/
+.vscode
+/.project
+*.log
+
+/node_modules/
+*.js.ignore
+