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/src/js/actions/_README.md

@@ -0,0 +1,5 @@
+# Managing Undo History with Actions
+
+More information on wiki page:
+
+https://github.com/viliusle/miniPaint/wiki/Undo-Redo-system