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/session_store.py

@@ -69,6 +69,15 @@ def save(session_id: str, messages: list[dict]) -> None:
     }, indent=2))
 
 
+def delete(session_id: str) -> bool:
+    """Delete a session file. Returns True if it existed and was deleted."""
+    path = _path(session_id)
+    if not path.exists():
+        return False
+    path.unlink()
+    return True
+
+
 def list_all() -> list[dict]:
     d = settings.sessions_path()
     if not d.exists():