feat: complete phase 1 -- core framework with chat loop, agents, and React UI

Backend:
- FastAPI WebSocket /ws endpoint with streaming via LangGraph astream
- LangGraph Supervisor connecting 3 mock agents (order_lookup, order_actions, fallback)
- YAML Agent Registry with Pydantic validation and immutable configs
- PostgresSaver checkpoint persistence via langgraph-checkpoint-postgres
- Session TTL with 30-min sliding window and interrupt extension
- LLM provider abstraction (Anthropic/OpenAI/Google)
- Token usage + cost tracking callback handler
- Input validation: message size cap, thread_id format, content length
- Security: no hardcoded defaults, startup API key validation, no input reflection

Frontend:
- React 19 + TypeScript + Vite chat UI
- WebSocket hook with reconnect + exponential backoff
- Streaming token display with agent attribution
- Interrupt approval/reject UI for write operations
- Collapsible tool call viewer

Testing:
- 87 unit tests, 87% coverage (exceeds 80% requirement)
- Ruff lint + format clean

Infrastructure:
- Docker Compose (PostgreSQL 16 + backend)
- pyproject.toml with full dependency management

backend/tests/unit/test_main.py

@@ -0,0 +1,27 @@
+"""Tests for app.main module."""
+
+from __future__ import annotations
+
+import pytest
+
+from app.main import AGENTS_YAML, FRONTEND_DIST, app
+
+
+@pytest.mark.unit
+class TestMainModule:
+    def test_app_title(self) -> None:
+        assert app.title == "Smart Support"
+
+    def test_app_version(self) -> None:
+        assert app.version == "0.1.0"
+
+    def test_agents_yaml_path_exists(self) -> None:
+        assert AGENTS_YAML.name == "agents.yaml"
+
+    def test_frontend_dist_path(self) -> None:
+        assert "frontend" in str(FRONTEND_DIST)
+        assert "dist" in str(FRONTEND_DIST)
+
+    def test_websocket_route_registered(self) -> None:
+        routes = [r.path for r in app.routes if hasattr(r, "path")]
+        assert "/ws" in routes