refactor(frontend): port search-zen-50 style to Go templates

Replace React SPA with simple Go templates using search-zen-50
visual style. No JavaScript required - pure HTML/CSS with clean
teal accent color scheme, monospace logo, and minimal design.

- Simplified base.html without HTMX or autocomplete JS
- Clean homepage with centered search box
- Results page with sticky header and category tabs
- Simplified CSS matching search-zen-50 aesthetics

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/kafka/main.go

@@ -19,6 +19,7 @@ package main
 import (
 	"flag"
 	"fmt"
+	"io/fs"
 	"log"
 	"log/slog"
 	"net/http"
@@ -30,7 +31,7 @@ import (
 	"github.com/metamorphosis-dev/kafka/internal/httpapi"
 	"github.com/metamorphosis-dev/kafka/internal/middleware"
 	"github.com/metamorphosis-dev/kafka/internal/search"
-	"github.com/metamorphosis-dev/kafka/internal/spa"
+	"github.com/metamorphosis-dev/kafka/internal/views"
 )
 
 func main() {
@@ -80,15 +81,23 @@ func main() {
 
 	mux := http.NewServeMux()
 
-	// API routes - handled by Go
-	mux.HandleFunc("/healthz", h.Healthz)
+	// HTML template routes
+	mux.HandleFunc("/", h.Index)
 	mux.HandleFunc("/search", h.Search)
+	mux.HandleFunc("/preferences", h.Preferences)
+
+	// API routes
+	mux.HandleFunc("/healthz", h.Healthz)
 	mux.HandleFunc("/autocompleter", h.Autocompleter)
 	mux.HandleFunc("/opensearch.xml", h.OpenSearch(cfg.Server.BaseURL))
 
-	// SPA handler - serves React app for all other routes
-	spaHandler := spa.NewHandler()
-	mux.Handle("/", spaHandler)
+	// Serve embedded static files (CSS, JS, images).
+	staticFS, err := views.StaticFS()
+	if err != nil {
+		log.Fatalf("failed to load static files: %v", err)
+	}
+	var subFS fs.FS = staticFS
+	mux.Handle("/static/", http.StripPrefix("/static/", http.FileServer(http.FS(subFS))))
 
 	// Apply middleware: global rate limit → burst rate limit → per-IP rate limit → CORS → security headers → handler.
 	var handler http.Handler = mux