feat(webui): display context usage

[Premsenareddy]

Summary

• persist per-session context usage in AgentState, including current tokens, compression threshold, model window, and trigger ratio
• refresh context usage during compression/model reasoning and publish it through the existing state_updated SSE path
• add a compact Web UI context usage indicator in the chat toolbar, initialized from SessionRecord.state and updated live from SSE/model usage events

How the frontend stays updated

• Initial value comes from the existing sessions response: session.state.context_usage
• Real-time authoritative updates arrive through the existing /sessions/{id}/stream SSE channel as CustomEvent(name="state_updated") with context_usage included
• During an active reply, MODEL_CALL_END updates the displayed current token count immediately; the final state_updated event reconciles it with the persisted backend estimate

Tests

• .venv/bin/python -m pytest -p no:cacheprovider tests/compress_context_test.py tests/agent_basic_test.py tests/service_wakeup_dispatcher_test.py tests/service_inbox_middleware_test.py tests/service_team_tools_test.py tests/storage_redis_test.py -q
• corepack pnpm --dir examples/web_ui/frontend build
• corepack pnpm --dir examples/web_ui/frontend lint
• Browser smoke: opened http://127.0.0.1:5173/chat and checked for console errors

Addresses #1845