[P2] Confirm or undo individual chart deletion #357

Priority

P2 - a single accidental action permanently removes a chart and its board placement, then autosaves the loss.

Finding

ChartGrid.tsx and ChartList.tsx call onRemove immediately. There is no confirmation or undo carrying the complete chart and placement data.

Acceptance criteria

• Require explicit confirmation or provide an undo that restores full chart data and the previous board position.
• Ensure cancel or undo survives the autosave path correctly.
• Add tests for deletion, cancellation, and restoration.