BeszelAreaChart hardcoded type="stacked" for all charts, causing Docker
Network I/O to render blended colors that didn't match the tooltip.
- Make `type` prop configurable (default "default")
- Only stack Memory, Docker CPU, and Docker Memory charts
- Add systemId as a persisted widget option with Menu-based selector
- Replace Popover with Menu trigger="click-hover" for proper hover UX
- Show error state with Select when selected system is not found
- Remove 1-minute time period (live SSE not wired yet)