Add thinking/reasoning streaming support ...

Enable Ollama think param and stream reasoning chunks to client.
New agent events: ThinkingDelta, ThinkingEnd. Config gains ollama_think
and ollama_num_ctx settings. WebSocket protocol updated accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Persistent sessions and client module refactor ...

Server:
- SqliteSessionStore replaces InMemorySessionStore as default backend
- Sessions survive server restarts (stored in navi.db)
- DB_PATH configurable via .env

Client:
- Split monolithic app.js into ES modules:
  js/api.js    — REST calls
  js/ws.js     — WebSocket wrapper (WsClient class)
  js/chat.js   — message area DOM helpers
  js/sidebar.js — session list and header helpers
  js/app.js    — state, wiring, boot
- Active session persisted in localStorage — restored on page reload

Initial implementation of the agent system core ...

- FastAPI server with REST API and WebSocket streaming
- Modular LLM backend abstraction (Ollama implemented, OpenAI stub)
- Tool system: web_search (ddgs), filesystem, http_request, code_exec, terminal
- Agent profiles: smart_home, server_admin, secretary
- Tool-calling loop with concurrent tool execution
- In-memory session store with SessionStore ABC for future persistence
- Registry pattern for tools, profiles, and backends
- Orchestrator stub as foundation for multi-agent scenarios