feat: add embedded web UI

Adds an optional web dashboard served by OpenGFW itself, enabled with a new
`web` section in the config file.

Backend (web package, decoupled from engine/io so it builds on any OS):
- hub.go collects statistics off the engine logger callbacks: atomic counters,
  two ring-buffered time series (10s and 1min buckets), top N hosts/blocked
  destinations/rules/analyzers, and a 512 entry event buffer fanned out to
  connected clients over SSE. Slow clients drop frames instead of blocking
  the engine.
- api.go exposes /api/v1 for info, meta, metrics, events, the SSE stream and
  ruleset read/validate/replace.
- auth.go implements password login with in-memory session tokens and login
  rate limiting. Mutating endpoints require the bearer token (the session
  cookie is only accepted for GET), which makes them CSRF-safe.
- cmd/web.go implements the rule manager: rules are compiled before anything
  is written, the file is replaced atomically and the engine is hot reloaded.
  The SIGHUP handler now shares that same path.
- web/devserver serves the UI with synthetic traffic for frontend work on
  machines where the engine itself cannot be built.

Frontend (web/frontend, Vue 3 + Vite + Tailwind CSS v4 + Reka UI):
dashboard, live event feed with analyzer property inspection, visual and YAML
rule editors, analyzer overview and settings. Responsive down to phone sizes
with a bottom tab bar and bottom-sheet dialogs, plus light/dark themes and
English/Chinese translations.

The built UI in web/dist is committed and embedded with go:embed so that
`go build` works without Node; CI builds the frontend and checks that the
committed output is up to date.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/frontend/vite.config.ts

@@ -0,0 +1,29 @@
+import { fileURLToPath, URL } from 'node:url'
+
+import tailwindcss from '@tailwindcss/vite'
+import vue from '@vitejs/plugin-vue'
+import { defineConfig } from 'vite'
+
+// The production build lands in ../dist, which is embedded into the Go binary.
+export default defineConfig({
+  plugins: [vue(), tailwindcss()],
+  resolve: {
+    alias: {
+      '@': fileURLToPath(new URL('./src', import.meta.url)),
+    },
+  },
+  build: {
+    outDir: '../dist',
+    emptyOutDir: true,
+    chunkSizeWarningLimit: 900,
+  },
+  server: {
+    port: 5173,
+    proxy: {
+      '/api': {
+        target: process.env.OPENGFW_API ?? 'http://127.0.0.1:8080',
+        changeOrigin: true,
+      },
+    },
+  },
+})