Fix settings route switching and document API token system ...

- App.vue: make route reactive via ref + getRouteFromHash() so
  #settings toggles work without page reload
- docs/api_tokens.md: new comprehensive API token auth doc
- docs/api.md: add /api-tokens REST endpoints
- docs/auth.md: add token flow architecture diagram and resolution order
- docs/websocket.md: add auth section with cookie vs query-param token
- docs/architecture.md: update to AgentSessionOrchestrator + ToolContext
- docs/tools.md: add gnexus-creds MCP tools
- docs/index.md: link to api_tokens.md
- Rebuild webclient dist

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