New maglev-frontend component; promote LB sync events to INFO

Introduces maglev-frontend, a responsive, real-time web dashboard for one
or more running maglevd instances. Source lives at cmd/frontend/; the
built binary is maglev-frontend. It is a single Go process with the
SolidJS SPA embedded via //go:embed — no runtime file dependencies.

Architecture
 - One persistent gRPC connection per configured maglevd (-server A,B,C).
   Each connection runs three background loops: a WatchEvents stream
   subscribed at log_level=debug for live events, a 30s refresh loop as
   a safety net for drift, and a 5s health loop that surfaces connection
   drops quickly.
 - In-process pub/sub broker with a 30s / 2000-event replay ring using
   <epoch>-<seq> monotonic IDs. Short browser reconnects (nginx idle,
   wifi flap, laptop wake) silently replay buffered events via the
   EventSource Last-Event-ID header; longer outages or frontend restarts
   fall through to a "resync" event that triggers a full state refetch.
 - HTTP surface: /view/ (SPA), /view/api/state, /view/api/state/{name},
   /view/api/maglevds, /view/api/version, /view/api/events (SSE),
   /healthz, and an /admin/* placeholder returning 501 for a future
   basic-auth mutation surface.
 - SSE handler follows the full operational checklist: retry hint, 15s
   : ping heartbeat, Flush after every write, r.Context().Done() teardown,
   X-Accel-Buffering: no, and no gzip.

SolidJS SPA (cmd/frontend/web/, Vite + TypeScript)
 - solid-js/store for a reactive per-maglevd state tree; reducers apply
   backend transitions, maglevd-status flips, and resync refetches.
 - Scope selector tabs for multi-maglevd support, per-maglevd frontend
   cards with pool tables showing state, configured weight, effective
   weight, and last-transition age.
 - ProbeHeartbeat component turns a middle-dot into ❤️ on probe-start and
   back on probe-done, driven by real log events; fixed-size wrapper so
   the emoji swap doesn't jiggle the row.
 - Flash wrapper animates any primitive on change (1s yellow fade via
   Web Animations API, skipped on first mount). Wired into the state
   badge, configured weight, and effective weight columns.
 - DebugPanel: chronological rolling event tail with tail-style auto-
   scroll, pause/resume, and scope/firehose filter. Syntactic highlight
   for vpp-lb-sync-* events with fixed-order attribute formatting.
 - Live effective_weight updates: vpp-lb-sync-as-added/removed/weight-
   updated log events are routed through a reducer that walks the
   snapshot's pool rows and sets effective_weight on every match
   without waiting for the 30s refresh.
 - Header shows build version + commit with build date in a tooltip,
   fetched once from /view/api/version on mount.
 - Prettier wired in as the web-side fixstyle; make fixstyle now tidies
   both Go and web in one shot via a new fixstyle-web target.

Per-mutation VPP LB sync logging
 - Promotes the addVIP/delVIP/addAS/delAS/setASWeight helpers from
   slog.Debug to slog.Info and renames them from vpp-lbsync-* to
   vpp-lb-sync-{vip-added,vip-removed,as-added,as-removed,as-weight-
   updated}. Matching rename for vpp-lb-sync-start / -done / -error /
   -vip-recreate. The Prometheus metric name (maglev_vpp_lbsync_total)
   is left alone to preserve dashboards.
 - setASWeight now takes the prior weight so the event can emit
   from=X to=Y and the UI can show the delta.
 - The vip field in every event is the bare address (no /32 or /128
   mask), matching the CLI output style.
 - Any listener on the gRPC WatchEvents stream — CLI watch events or
   maglev-frontend — now sees every VIP/AS dataplane change in real
   time without needing to raise the log level.

Build and tooling
 - Makefile: maglev-frontend added to BINARIES; build / build-amd64 /
   build-arm64 emit the binary alongside maglevd and maglevc. A new
   maglev-frontend-web target rebuilds the SolidJS bundle via npm.
 - web/dist/ is tracked so a bare `go build` keeps working for Go-only
   contributors and CI.
 - .gitignore skips cmd/frontend/web/node_modules/.

Stability fixes
 - maglevd's WatchEvents synthetic replay events (from==to, at_unix_ns=0)
   were corrupting the frontend's LastTransition cache with at=0,
   rendering as "20555d ago" in the browser. Client now skips synthetic
   events: the cache comes from refreshAll and doesn't need them.
 - Frontends, Backends, and HealthChecks are now served in the order
   returned by the corresponding List* RPC instead of Go map iteration
   order, so reloads and refreshes keep the SPA stable.

cmd/frontend/main.go

@@ -0,0 +1,126 @@
+// Copyright (c) 2026, Pim van Pelt <pim@ipng.ch>
+
+package main
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"os"
+	"os/signal"
+	"strings"
+	"syscall"
+	"time"
+
+	buildinfo "git.ipng.ch/ipng/vpp-maglev/cmd"
+)
+
+func main() {
+	if err := run(); err != nil {
+		slog.Error("startup-fatal", "err", err)
+		os.Exit(1)
+	}
+}
+
+func run() error {
+	printVersion := flag.Bool("version", false, "print version and exit")
+	servers := stringFlag("server", "", "MAGLEV_SERVERS", "comma-separated maglevd gRPC addresses (required)")
+	listen := stringFlag("listen", ":8080", "MAGLEV_LISTEN", "HTTP listen address")
+	logLevel := stringFlag("log-level", "info", "MAGLEV_LOG_LEVEL", "log verbosity (debug|info|warn|error)")
+	flag.Parse()
+
+	if *printVersion {
+		fmt.Printf("maglev-frontend %s (commit %s, built %s)\n",
+			buildinfo.Version(), buildinfo.Commit(), buildinfo.Date())
+		return nil
+	}
+
+	var level slog.Level
+	if err := level.UnmarshalText([]byte(*logLevel)); err != nil {
+		return fmt.Errorf("invalid log level %q: %w", *logLevel, err)
+	}
+	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: level})))
+	slog.Info("starting",
+		"version", buildinfo.Version(),
+		"commit", buildinfo.Commit(),
+		"date", buildinfo.Date())
+
+	addrs := parseServers(*servers)
+	if len(addrs) == 0 {
+		return errors.New("at least one -server address is required")
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	broker := NewBroker()
+
+	clients := make([]*maglevClient, 0, len(addrs))
+	for _, addr := range addrs {
+		c, err := newMaglevClient(addr, broker)
+		if err != nil {
+			return fmt.Errorf("connect %s: %w", addr, err)
+		}
+		clients = append(clients, c)
+		c.Start(ctx)
+		slog.Info("maglevd-configured", "name", c.name, "address", c.address)
+	}
+
+	mux := http.NewServeMux()
+	registerHandlers(mux, clients, broker)
+
+	srv := &http.Server{
+		Addr:              *listen,
+		Handler:           mux,
+		ReadHeaderTimeout: 10 * time.Second,
+	}
+
+	sigCh := make(chan os.Signal, 1)
+	signal.Notify(sigCh, syscall.SIGTERM, syscall.SIGINT)
+
+	errCh := make(chan error, 1)
+	go func() {
+		slog.Info("http-listening", "addr", *listen)
+		if err := srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
+			errCh <- err
+		}
+	}()
+
+	select {
+	case sig := <-sigCh:
+		slog.Info("shutdown", "signal", sig)
+	case err := <-errCh:
+		cancel()
+		return err
+	}
+
+	cancel()
+	shutdownCtx, shutdownCancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer shutdownCancel()
+	_ = srv.Shutdown(shutdownCtx)
+	for _, c := range clients {
+		c.Close()
+	}
+	return nil
+}
+
+func parseServers(s string) []string {
+	var out []string
+	for _, part := range strings.Split(s, ",") {
+		if p := strings.TrimSpace(part); p != "" {
+			out = append(out, p)
+		}
+	}
+	return out
+}
+
+func stringFlag(name, defaultVal, envKey, usage string) *string {
+	val := defaultVal
+	if v := os.Getenv(envKey); v != "" {
+		val = v
+	}
+	return flag.String(name, val, fmt.Sprintf("%s (env: %s)", usage, envKey))
+}