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/src/components/ui/TabsContent.vue

@@ -0,0 +1,13 @@
+<script setup lang="ts">
+import { TabsContent } from 'reka-ui'
+
+import { cn } from '@/lib/utils'
+
+const props = defineProps<{ class?: string; value: string }>()
+</script>
+
+<template>
+  <TabsContent :value="props.value" :class="cn('outline-none', props.class)">
+    <slot />
+  </TabsContent>
+</template>