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/web/src/App.tsx

@@ -0,0 +1,62 @@
+import { createSignal, onMount, type Component } from "solid-js";
+import { fetchAllState, fetchVersion } from "./api/rest";
+import { openEventStream } from "./api/sse";
+import { replaceAll, state } from "./stores/state";
+import { scope, setScope } from "./stores/scope";
+import ScopeSelector from "./components/ScopeSelector";
+import Overview from "./views/Overview";
+import DebugPanel from "./views/DebugPanel";
+import type { VersionInfo } from "./types";
+
+const isAdmin = window.location.pathname.startsWith("/admin");
+
+const App: Component = () => {
+  const [error, setError] = createSignal<string | undefined>();
+  const [version, setVersion] = createSignal<VersionInfo | undefined>();
+
+  onMount(async () => {
+    try {
+      const [snaps, ver] = await Promise.all([fetchAllState(), fetchVersion()]);
+      replaceAll(snaps);
+      setVersion(ver);
+      if (!scope() && snaps.length > 0) {
+        setScope(snaps[0].maglevd.name);
+      }
+      openEventStream();
+    } catch (err) {
+      setError(`${err}`);
+    }
+  });
+
+  return (
+    <div class="app">
+      <header class="app-header">
+        <div class="brand">
+          <strong>maglev</strong>
+          {version() && (
+            <span class="version" title={`commit ${version()!.commit} · built ${version()!.date}`}>
+              {version()!.version} ({version()!.commit})
+            </span>
+          )}
+        </div>
+        <ScopeSelector />
+        <span class="mode-tag">{isAdmin ? "admin" : "view"}</span>
+        <a
+          class="admin-toggle"
+          href={isAdmin ? "/view/" : "/admin/"}
+          title={isAdmin ? "exit admin mode" : "enter admin mode"}
+        >
+          {isAdmin ? "exit admin" : "admin…"}
+        </a>
+      </header>
+
+      {error() && <div class="banner err">{error()}</div>}
+      {!error() && Object.keys(state.byName).length === 0 && <p class="loading">Loading…</p>}
+
+      <Overview />
+      <DebugPanel />
+    </div>
+  );
+};
+
+export default App;