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/package.json

@@ -0,0 +1,31 @@
+{
+  "name": "opengfw-web",
+  "version": "1.0.0",
+  "private": true,
+  "type": "module",
+  "description": "Web UI for OpenGFW",
+  "scripts": {
+    "dev": "vite",
+    "build": "vue-tsc -b && vite build",
+    "preview": "vite preview",
+    "typecheck": "vue-tsc -b --noEmit"
+  },
+  "dependencies": {
+    "class-variance-authority": "^0.7.1",
+    "clsx": "^2.1.1",
+    "lucide-vue-next": "^0.469.0",
+    "reka-ui": "^2.5.0",
+    "tailwind-merge": "^3.3.1",
+    "vue": "^3.5.13",
+    "vue-router": "^4.5.0"
+  },
+  "devDependencies": {
+    "@tailwindcss/vite": "^4.1.11",
+    "@types/node": "^22.10.5",
+    "@vitejs/plugin-vue": "^5.2.1",
+    "tailwindcss": "^4.1.11",
+    "typescript": "~5.7.3",
+    "vite": "^6.0.7",
+    "vue-tsc": "^2.2.0"
+  }
+}