Add update-vendor.sh symlink support, unify install.sh vendor flow

- update-vendor.sh now creates webui/vendor symlinks (htm.js)
- install.sh calls update-vendor.sh after package install
- Add vendor/.empty and webui/vendor/.empty as directory placeholders in git

docs/overview.md

@@ -136,7 +136,7 @@ After installation, access the management interface at `https://<hostname>.local
 │   ├── config.md
 │   └── hoover.md            # Hoover SPA framework
 └── scripts/                   # Utility scripts
-    └── update-vendor.sh      # Download acme.sh binary
+    └── update-vendor.sh      # Download vendored libraries (acme.sh, htm)
 ```
 
 ## Documentation