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.

hermes_cli/main.py

@@ -12022,13 +12022,14 @@ def cmd_dashboard(args):
 
     from hermes_cli.web_server import start_server
 
-    embedded_chat = args.tui or os.environ.get("HERMES_DASHBOARD_TUI") == "1"
+    # The in-browser Chat tab (the embedded TUI over PTY/WebSocket) is always
+    # available — the desktop app and the dashboard's own Chat tab both rely on
+    # the `/api/ws` + `/api/pty` sockets, so there is no reason to gate them.
     start_server(
         host=args.host,
         port=args.port,
         open_browser=not args.no_open,
         allow_public=getattr(args, "insecure", False),
-        embedded_chat=embedded_chat,
     )
 
 
@@ -15314,14 +15315,6 @@ Examples:
         action="store_true",
         help="Allow binding to non-localhost (DANGEROUS: exposes API keys on the network)",
     )
-    dashboard_parser.add_argument(
-        "--tui",
-        action="store_true",
-        help=(
-            "Expose the in-browser Chat tab (embedded `hermes --tui` via PTY/WebSocket). "
-            "Alternatively set HERMES_DASHBOARD_TUI=1."
-        ),
-    )
     dashboard_parser.add_argument(
         "--skip-build",
         action="store_true",