feat: Add task launcher API with timestamp-based run directories

- Add POST /runtime/start, /stop, /restart APIs
- Run directories use timestamp format: YYYYMMDD_HHMMSS
- Add force stop support in TradingRuntimeManager
- Frontend: use REST API to start runtime instead of WebSocket
- Remove RuntimeView page from navigation

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

backend/runtime/manager.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 import json
 from datetime import datetime, UTC
 from pathlib import Path
@@ -10,6 +11,7 @@ from .context import TradingRunContext
 from .registry import RuntimeRegistry
 
 _global_runtime_manager: Optional["TradingRuntimeManager"] = None
+_shutdown_event: Optional[asyncio.Event] = None
 
 
 def set_global_runtime_manager(manager: "TradingRuntimeManager") -> None:
@@ -26,6 +28,28 @@ def get_global_runtime_manager() -> Optional["TradingRuntimeManager"]:
     return _global_runtime_manager
 
 
+def set_shutdown_event(event: asyncio.Event) -> None:
+    """Set the global shutdown event for signaling runtime stop."""
+    global _shutdown_event
+    _shutdown_event = event
+
+
+def clear_shutdown_event() -> None:
+    """Clear the global shutdown event."""
+    global _shutdown_event
+    _shutdown_event = None
+
+
+def get_shutdown_event() -> Optional[asyncio.Event]:
+    """Get the global shutdown event if set."""
+    return _shutdown_event
+
+
+def is_shutdown_requested() -> bool:
+    """Check if shutdown has been requested."""
+    return _shutdown_event is not None and _shutdown_event.is_set()
+
+
 class TradingRuntimeManager:
     def __init__(self, config_name: str, run_dir: Path, bootstrap: Optional[Dict[str, Any]] = None) -> None:
         self.config_name = config_name