feat(dashboard): always enable embedded chat; remove dashboard --tui flag

The dashboard's embedded Chat surface (/chat, /api/ws, /api/pty) was gated
behind `hermes dashboard --tui` / HERMES_DASHBOARD_TUI=1. The desktop app and
the dashboard's own Chat tab both drive the agent over the /api/ws + /api/pty
WebSockets, so a dashboard started without the flag would pass the /api/status
health check but slam the chat WebSocket shut with WS code 4403 — the app
connects, reports "ready", and chat stays dead. This was the root cause behind
multiple user reports of the desktop app failing to connect to a self-hosted
gateway/dashboard, and it bit Docker and host installs alike.

Make the embedded chat unconditional:

- web_server.py: _DASHBOARD_EMBEDDED_CHAT_ENABLED defaults to True; drop the
  embedded_chat parameter and the runtime reassignment from start_server().
  The WS gates still read the constant (now always true) so the seam — and its
  "rejects when disabled" contract test — stays meaningful.
- main.py: remove the `--tui` argument from the dashboard subparser and the
  `embedded_chat = args.tui or HERMES_DASHBOARD_TUI==1` derivation.
- web/: isDashboardEmbeddedChatEnabled() returns true unconditionally; drop the
  deprecated __HERMES_DASHBOARD_TUI__ alias and the dead LEGACY_TUI_RE scrape in
  the vite dev-token plugin.
- apps/desktop/electron/main.cjs: drop `--tui` from the spawned dashboardArgs
  (it would now error with "unrecognized arguments: --tui") and the redundant
  HERMES_DASHBOARD_TUI env injection.
- Docker: no s6 run-script change needed — the script never passed --tui; the
  HERMES_DASHBOARD_TUI env var is now simply a no-op, so the image works out of
  the box with no extra var.
- Docs: remove every dashboard --tui / HERMES_DASHBOARD_TUI reference across the
  CLI reference, env-var reference, docker/desktop/web-dashboard guides, in-app
  tips, and the zh-Hans translations. The terminal `hermes --tui` / HERMES_TUI
  references are intentionally left untouched.

Tests: 270 passing across web_server, dashboard lifecycle, host-header,
auth-gate, and docker-override-scripts suites.

website/i18n/zh-Hans/docusaurus-plugin-content-docs/current/user-guide/features/web-dashboard.md

@@ -24,7 +24,6 @@ hermes dashboard
 | `--host` | `127.0.0.1` | 绑定地址 |
 | `--no-open` | — | 不自动打开浏览器 |
 | `--insecure` | 关闭 | 允许绑定到非 localhost 主机（**危险**——会在网络上暴露 API 密钥；请配合防火墙和强认证使用） |
-| `--tui` | 关闭 | 启用浏览器内 Chat 标签页（通过 PTY/WebSocket 嵌入 `hermes --tui`）。也可设置 `HERMES_DASHBOARD_TUI=1`。 |
 
 ```bash
 # 自定义端口
@@ -35,9 +34,6 @@ hermes dashboard --host 0.0.0.0
 
 # 启动时不打开浏览器
 hermes dashboard --no-open
-
-# 启用浏览器内 Chat 标签页
-hermes dashboard --tui
 ```
 
 ## 前置条件
@@ -52,7 +48,7 @@ pip install 'hermes-agent[web,pty]'
 
 在没有依赖项的情况下运行 `hermes dashboard` 时，它会告诉你需要安装什么。如果前端尚未构建且 `npm` 可用，则会在首次启动时自动构建。
 
-Chat 标签页在普通 `hermes dashboard` 启动时默认关闭。如需嵌入式浏览器聊天面板，请使用 `hermes dashboard --tui` 启动，或设置 `HERMES_DASHBOARD_TUI=1`。
+Chat 标签页是每次 `hermes dashboard` 启动的一部分——内嵌的浏览器聊天面板（通过 PTY/WebSocket 运行 TUI）始终可用，无需任何额外参数。
 
 ## 页面