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/images/icons/italic.svg

@@ -0,0 +1,2 @@
+<?xml version="1.0" encoding="iso-8859-1"?>
+<svg width="1em" height="1em" viewBox="0 0 16 16" class="bi bi-type-italic" fill="currentColor" xmlns="http://www.w3.org/2000/svg"><path d="M7.991 11.674L9.53 4.455c.123-.595.246-.71 1.347-.807l.11-.52H7.211l-.11.52c1.06.096 1.128.212 1.005.807L6.57 11.674c-.123.595-.246.71-1.346.806l-.11.52h3.774l.11-.52c-1.06-.095-1.129-.211-1.006-.806z"/></svg>