Phase 3: PluginWebMixin SDK + web plugin control panels

- New: sdk/plugin_web.py (PluginWebMixin: web pages, API routes, SSE)
- New: services/web_panel/routes/plugin_web.py (3 endpoints)
- New: plugins/example_plugin/dashboard.html (counter + SSE demo)
- Updated: example_plugin uses PluginWebMixin
- Tests: 28/28 passing

services/web_panel/manager.py

@@ -5,7 +5,7 @@ import os
 import logging
 from pathlib import Path
 from aiohttp import web
-from .routes import auth, status, plugins, commands, logs, projects, proxy
+from .routes import auth, status, plugins, commands, logs, projects, proxy, plugin_web
 
 logger = logging.getLogger(__name__)
 
@@ -64,6 +64,7 @@ class WebPanelManager:
         logs.setup_routes(app, self.base_path)
         projects.setup_project_routes(app, self.sm)
         proxy.setup_proxy_routes(app, self.sm)
+        plugin_web.setup_plugin_web_routes(app, self.sm)
         
         # 注册日志广播
         ls = self.sm.get_service("log")