feat(spa): add SPA Go package with embedded dist FS

Creates internal/spa package that:
- Embeds React build output from cmd/kafka/dist/
- Provides HTTP handler for static file serving
- Falls back to index.html for SPA client-side routing

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

internal/spa/spa.go

@@ -0,0 +1,56 @@
+package spa
+
+import (
+	"embed"
+	"io/fs"
+	"net/http"
+	"path"
+)
+
+//go:embed all:dist
+var distFS embed.FS
+
+// DistFS returns the embedded dist directory as an fs.FS.
+func DistFS() (fs.FS, error) {
+	return fs.Sub(distFS, "dist")
+}
+
+// NewHandler returns an HTTP handler that:
+// - Serves static files from the embedded dist/ directory
+// - Falls back to index.html for SPA routing (any non-API path)
+func NewHandler() http.Handler {
+	dist, err := DistFS()
+	if err != nil {
+		panic("spa: embedded dist not found: " + err.Error())
+	}
+	return &spaHandler{dist: dist}
+}
+
+type spaHandler struct {
+	dist fs.FS
+}
+
+func (h *spaHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	// API paths are handled by Go API handlers - this should never be reached
+	// since Go mux dispatches to specific handlers first. But if reached,
+	// pass through to FileServer which will return 404 for unknown paths.
+
+	// Try to serve the requested file first
+	filePath := path.Clean(r.URL.Path)
+	f, err := h.dist.Open(filePath)
+	if err == nil {
+		f.Close()
+		// File exists - serve it via FileServer
+		http.FileServer(http.FS(h.dist)).ServeHTTP(w, r)
+		return
+	}
+
+	// Fallback to index.html for SPA routing
+	indexFile, err := h.dist.Open("index.html")
+	if err != nil {
+		http.Error(w, "index.html not found in embedded files", http.StatusInternalServerError)
+		return
+	}
+	indexFile.Close()
+	http.FileServer(http.FS(h.dist)).ServeHTTP(w, r)
+}