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.

tests/docker/test_dashboard.py

@@ -389,7 +389,7 @@ def test_dashboard_insecure_env_var_opts_out_of_gate(
     """``HERMES_DASHBOARD_INSECURE=1`` re-enables the legacy no-gate mode
     for operators running on trusted LANs behind a reverse proxy without
     the OAuth contract. Same opt-out shape as the rest of the s6 boolean
-    envs (``HERMES_DASHBOARD``, ``HERMES_DASHBOARD_TUI``).
+    envs (e.g. ``HERMES_DASHBOARD``).
 
     With the gate off, ``/api/status`` (a public endpoint under the
     legacy ``_SESSION_TOKEN`` middleware) returns 200 with the