refactor: 移除 FakeMessagePlatform channel 全部逻辑、展示、配置

- 删除 channels/fake/ 整个 Node.js FakeMessagePlatform (8文件)
- 删除后端 FakeProvider (fake.go + fake_test.go) + FakeWebhookHandler (fake_webhook.go)
- bootstrap.go: 移除 FakeProvider 接线 + isFakeChannelEnabled()
- router.go: 移除 FakeWebhook 字段 + /webhooks/fake/:identifier 路由
- provider.go: 移除 ChannelFake 常量
- inbox_service.go: 移除 fake channel 类型验证 + isFakeChannelAllowed()
- 前端: 移除 Fake.vue + ChannelFactory/ChannelList/ChannelItem 引用 + i18n (en/zh_CN)
- package.json: 移除 fake:start/dev/test/dev:all 脚本
- pnpm-workspace.yaml: 移除 channels/fake
- .env/.env.example/quickstart: 移除 GOCHAT_ALLOW_FAKE_CHANNEL
- 测试: bridge_listener_test 改用 web_widget, router_test 移除 fake 路由断言
- 保留 fake:ai LLM Provider (backend/internal/llm/fake_provider.go) — 与 channel 无关

package.json

@@ -7,10 +7,6 @@
     "dev:backend": "cd backend && air -c .air.toml",
     "dev:frontend": "cd frontend && pnpm dev",
     "dev": "concurrently -n backend,frontend -c blue,green \"pnpm dev:backend\" \"pnpm dev:frontend\"",
-    "fake:start": "cd channels/fake && tsx src/index.ts",
-    "fake:dev": "cd channels/fake && tsx watch src/index.ts",
-    "fake:test": "pnpm --dir channels/fake test",
-    "dev:all": "concurrently -n backend,frontend,fake -c blue,green,magenta \"pnpm dev:backend\" \"pnpm dev:frontend\" \"pnpm fake:start\"",
     "build:frontend": "cd frontend && pnpm build",
     "build:sdk": "cd frontend && pnpm build:sdk",
     "lint:frontend": "cd frontend && pnpm eslint",