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/Dockerfile.dev

@@ -0,0 +1,17 @@
+FROM node:20-alpine
+
+WORKDIR /app
+
+# Copy package files
+COPY package.json ./
+
+# Install dependencies
+RUN npm install
+
+# Copy source
+COPY . .
+
+EXPOSE 5173
+
+# Run dev server
+CMD ["npm", "run", "dev", "--", "--host"]