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

tests/test_server.py

@@ -66,7 +66,7 @@ class TestBlueprintsRegistered:
     def test_all_blueprints_registered(self, client):
         from webui.server import BLUEPRINTS
 
-        assert len(BLUEPRINTS) == 7
+        assert len(BLUEPRINTS) == 8
         names = [name for name, _ in BLUEPRINTS]
         assert "firewall" in names
         assert "network" in names