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>

README.md

@@ -33,7 +33,28 @@ Telegram group: https://t.me/OpGFW
 - Hot-reloadable rules (send `SIGHUP` to reload)
 - Flexible analyzer & modifier framework
 - Extensible IO implementation (only NFQueue for now)
-- [WIP] Web UI
+- Web UI with live traffic dashboard and rule editor
+
+## Web UI
+
+OpenGFW ships with an optional web dashboard: live traffic statistics, a real-time event
+feed with per-stream analyzer properties, and a rule editor that validates and hot reloads
+the ruleset. It works on phones as well as on desktops.
+
+Enable it in your config file:
+
+```yaml
+web:
+  enabled: true
+  listen: :8080
+  secret: your-password-here # generated and logged on startup if left empty
+  # cert: /path/to/fullchain.pem
+  # key: /path/to/privkey.pem
+```
+
+The UI is embedded in the binary, so no extra files need to be deployed. Only expose it to
+trusted networks — anyone with the password can change your rules. See
+[web/README.md](web/README.md) for development instructions.
 
 ## Use cases