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/html_handler.go

@@ -0,0 +1,34 @@
+package web
+
+import "net/http"
+
+// HTMLHandler serves the Go-native HTML frontend.
+type HTMLHandler struct {
+	renderer *Renderer
+	build    BuildInfo
+}
+
+// NewHTMLHandler constructs an HTMLHandler.
+func NewHTMLHandler(r *Renderer, b BuildInfo) *HTMLHandler {
+	return &HTMLHandler{renderer: r, build: b}
+}
+
+func (h *HTMLHandler) ServeAdults(w http.ResponseWriter, r *http.Request) {
+	h.renderer.Render(w, "adults", PageData{Active: "adults", Build: h.build})
+}
+
+func (h *HTMLHandler) ServeJuniors(w http.ResponseWriter, r *http.Request) {
+	h.renderer.Render(w, "juniors", PageData{Active: "juniors", Build: h.build})
+}
+
+func (h *HTMLHandler) ServePayments(w http.ResponseWriter, r *http.Request) {
+	h.renderer.Render(w, "payments", PageData{Active: "payments", Build: h.build})
+}
+
+func (h *HTMLHandler) ServeSync(w http.ResponseWriter, r *http.Request) {
+	h.renderer.Render(w, "sync", PageData{Active: "sync", Build: h.build})
+}
+
+func (h *HTMLHandler) ServeFlushCache(w http.ResponseWriter, r *http.Request) {
+	h.renderer.Render(w, "flush_cache", PageData{Active: "flush", Build: h.build})
+}