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/server.go

@@ -6,6 +6,7 @@ import (
 	"fuj-management/go/internal/services/membership"
 	"fuj-management/go/internal/web/api"
 	"fuj-management/go/internal/web/middleware"
+	"io/fs"
 	"log/slog"
 	"net/http"
 )
@@ -19,7 +20,12 @@ type BuildInfo struct {
 
 // Run registers routes and starts the HTTP server on addr.
 func Run(logger *slog.Logger, addr string, build BuildInfo, sources membership.Sources, cfg config.Config) error {
-	h := &api.Handler{
+	renderer, err := NewRenderer()
+	if err != nil {
+		return fmt.Errorf("init templates: %w", err)
+	}
+
+	ah := &api.Handler{
 		BuildVersion: build.Version,
 		BuildCommit:  build.Commit,
 		BuildDate:    build.BuildDate,
@@ -27,22 +33,34 @@ func Run(logger *slog.Logger, addr string, build BuildInfo, sources membership.S
 		Config:       cfg,
 		Logger:       logger,
 	}
+	hh := NewHTMLHandler(renderer, build)
+
+	staticSubFS, err := fs.Sub(staticFS, "static")
+	if err != nil {
+		return fmt.Errorf("static subfs: %w", err)
+	}
 
 	mux := http.NewServeMux()
-	mux.HandleFunc("GET /{$}", helloHandler(build))
-	mux.HandleFunc("GET /api/version", h.ServeVersion)
-	mux.HandleFunc("GET /api/adults", h.ServeAdults)
-	mux.HandleFunc("GET /api/juniors", h.ServeJuniors)
-	mux.HandleFunc("GET /api/payments", h.ServePayments)
+
+	// HTML routes
+	mux.HandleFunc("GET /{$}", func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, "/adults", http.StatusFound)
+	})
+	mux.HandleFunc("GET /adults", hh.ServeAdults)
+	mux.HandleFunc("GET /juniors", hh.ServeJuniors)
+	mux.HandleFunc("GET /payments", hh.ServePayments)
+	mux.HandleFunc("GET /sync-bank", hh.ServeSync)
+	mux.HandleFunc("GET /flush-cache", hh.ServeFlushCache)
+
+	// Static files
+	mux.Handle("/static/", http.StripPrefix("/static/", http.FileServerFS(staticSubFS)))
+
+	// JSON API routes
+	mux.HandleFunc("GET /api/version", ah.ServeVersion)
+	mux.HandleFunc("GET /api/adults", ah.ServeAdults)
+	mux.HandleFunc("GET /api/juniors", ah.ServeJuniors)
+	mux.HandleFunc("GET /api/payments", ah.ServePayments)
 
 	logger.Info("starting server", "addr", addr)
 	return http.ListenAndServe(addr, middleware.RequestTimer(logger, mux))
 }
-
-func helloHandler(build BuildInfo) http.HandlerFunc {
-	return func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
-		fmt.Fprintf(w, "fuj-go ok\nversion: %s\ncommit:  %s\nbuilt:   %s\n",
-			build.Version, build.Commit, build.BuildDate)
-	}
-}