Convert the Y2K theme from a Go+templ wasm plugin to a fully codeless .bnp:
blocks/blocks.yaml + .ninjatpl, page/system templates, template overrides,
email wrapper, master pages and CSS declared in manifest.yaml. All *.go, *.templ,
*_templ.go, go.mod and go.sum deleted; schemas moved schemas/ -> blocks/.
Static parity proven 48/48 identical against the templ baseline (10 blocks x
full+empty+variant branches, 4 overrides incl. heading L1-6, email full+empty).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
