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).

Fix session file URLs (add /api prefix) and WebGL error handling in STL viewer ...

Backend:
- _file_url in content_store.py now returns /api/sessions/... instead of /sessions/...
- share_file.py URL construction updated to include /api
- content_publish.py docstring updated to reflect correct endpoint

Frontend:
- contentLinks.js: avoid double /api prefix in dev mode when backend already returns it
- stl.html: replace throw err with return after showing WebGL fallback message
- Rebuild dist

Tests:
- Update expected URLs in test_content_store.py and test_share_file.py

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Add artifact source previews

Improve content publishing UX