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/vite.config.js

@@ -0,0 +1,20 @@
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+
+export default defineConfig({
+  plugins: [react()],
+  server: {
+    host: '0.0.0.0',
+    port: 5173,
+    proxy: {
+      '/projects': {
+        target: 'http://backend:8000',
+        changeOrigin: true,
+      },
+      '/edits': {
+        target: 'http://backend:8000',
+        changeOrigin: true,
+      },
+    },
+  },
+})