Add chart slot editor in the inspector

## Summary
`data-chart` layouts store chart data as JSON `{labels, values}`. The inspector has no dedicated editor.

## Tasks
- [ ] Detect `chart` slot type in inspector
- [ ] Add simple label/value row editor (or JSON textarea with validation)
- [ ] Preview updates live on canvas

## Files
- `apps/web/app/editor/[id]/Editor.tsx`
- `layouts/data-chart/slide.config.ts`
- `packages/ui/src/SlotRenderer.tsx`

Start by reading layouts/data-chart/slide.config.ts to confirm the chart slot shape, then trace how apps/web/app/editor/[id]/Editor.tsx and packages/ui/src/SlotRenderer.tsx handle inspector slots and canvas rendering. Done means chart slots have an editor with validation, and changing labels or values updates the canvas preview live.

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