port: 8000/8001 -> 8099/8098 everywhere ...

navi runs on shared servers where 8000/8001 are usually taken. New
defaults: API 8099, navi_ui MCP 8098. Touched: config defaults
(navi_port, navi_ui_mcp_port, public_url, gnauth_redirect_uri),
navi-server launcher docs, env.template/.env.example, install.sh
health-check fallback, terminal client base_url, webclient dev configs
(useWebSocket, contentLinks, vite proxy), android url hint, docs.
Also made the navi_ui FastMCP constructor port settings-driven instead
of a hardcoded 8001 (it was overridden at start anyway).

server: NAVI_WEBCLIENT_ENABLED gate + navi-server launcher ...

NAVI_WEBCLIENT_ENABLED=false removes every web-facing route (/, /assets,
/images, /content-viewers, /content, /admin panel, /debug*) and skips
the navi_ui MCP server — its only consumer is the webclient. The REST
API and WS the terminal client uses stay fully intact, and so does the
/admin/* JSON API (still gated by require_admin). Static mounts use
check_dir=False so a tree without webclient/dist imports fine.

New navi-server console script (navi/server.py): uvicorn launcher that
reads NAVI_HOST/NAVI_PORT from settings — the systemd unit needs no
hardcoded values.

Default stays true (full web UI), so nothing changes for dev setups;
subprocess route tests cover both modes.