Switch to datastar-pro and stop tracking downloaded libs

Datastar-pro is fetched from a private Gitea repo (ryan/vendor-libs)
using VENDOR_TOKEN for CI/Docker builds, with a local fallback from
../optional/ for development. DaisyUI is pinned to v5.5.19 instead of
tracking latest. Downloaded files are now gitignored and fetched at
build time via 'task download', which is a dependency of both build
and live tasks.

.gitignore

@@ -27,6 +27,10 @@
 *_templ.go
 assets/css/output.css
 
+# Downloaded client-side libs (fetched by cmd/downloader)
+assets/js/datastar/*
+assets/css/daisyui/*
+
 # Deploy scripts and configs
 !deploy/*.sh
 !deploy/*.service