chore: remove React frontend, SPA server, and compiled binary

The project uses pure Go HTML templates + CSS. The React frontend
(frontend/), SPA handler (internal/spa/), and prebuilt binary (kafka)
were dead weight.

Also removes the frontend replacement plan/spec docs.

internal/spa/spa.go

@@ -1,56 +0,0 @@
-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)
-}