feat(go): M6.1 — template skeleton, embed.FS, HTML routes

Stand up the Go-native HTML frontend foundation:
- base.tmpl layout + nav/footer partials (three-tier nav, active-link highlighting)
- terminal-green-on-black theme extracted to static/css/app.css (served via embed.FS)
- HTMLHandler with stub pages for all five routes; / redirects to /adults
- NewRenderer parses per-page template sets at startup so parse failures abort boot
- Smoke test: each route returns 200 text/html with exactly one class="active" link

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/web/templates/juniors.tmpl

@@ -0,0 +1,5 @@
+{{define "title"}}Juniors{{end}}
+{{define "content"}}
+<h1>Juniors Dashboard</h1>
+<p class="description">Coming in M6.3</p>
+{{end}}