feat: session delete + touch-friendly message controls

Session delete:
- DELETE /sessions/{session_id} endpoint (chat.py + session_store.py)
- × button on each session item in the panel (hover-reveal on desktop)
- Clears UI if the active session is deleted

Touch accessibility:
- @media (hover: none) rule makes msg-actions always visible on touch devices
- msg-act-btn tap targets enlarged to 36px min-height, readable font size
- session-delete-btn also always visible and finger-sized on touch

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

cortex/routers/chat.py

@@ -6,7 +6,7 @@ from pydantic import BaseModel
 from context_loader import load_context
 from llm_client import complete
 from session_logger import log_turn
-from session_store import load as load_session, save as save_session, list_all, generate_session_id
+from session_store import load as load_session, save as save_session, list_all, generate_session_id, delete as delete_session
 from config import settings
 import event_bus
 
@@ -143,6 +143,14 @@ async def list_sessions() -> dict:
     return {"sessions": list_all()}
 
 
+@router.delete("/sessions/{session_id}")
+async def delete_session_endpoint(session_id: str) -> dict:
+    found = delete_session(session_id)
+    if not found:
+        raise HTTPException(status_code=404, detail=f"Session {session_id} not found")
+    return {"ok": True, "session_id": session_id}
+
+
 @router.put("/history/{session_id}")
 async def replace_history(session_id: str, req: HistoryUpdate) -> dict:
     """Replace the full message list for a session (used by edit/delete UI)."""