Per-slice color and label overrides for pie/treemap/funnel

[TanayBensuYurtturk]

Adds a widget-level slices map ({label: {color, label}}) so pie, treemap, and funnel charts can recolor and rename individual slices, mirroring the existing per-series series map. Wired through the schema, Go model/loader/validator, TS types, and the chart renderer.

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
frontend/src/components/widgets/ChartWidget.tsx:1482
**Slice rename overwrites pie value**

When a pie chart uses the same result column for `label` and `value`, constructing `pieData` replaces the numeric value with the slice's display label before Recharts reads it, causing an invalid or zero-sized slice and incorrect percentages.

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "Add per-slice color and label overrides ..." | Re-trigger Greptile}

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
frontend/src/components/widgets/ChartWidget.tsx:1483
**Synthetic key overwrites pie value**

When a pie chart uses a result column named `__sliceName` as its value field, this object spread replaces the numeric value with the slice's display label before Recharts reads it, causing invalid slice sizes and percentages.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (2): Last reviewed commit: "Keep pie slice rename off the value colu..." | Re-trigger Greptile}

[greptile-apps]

_{Reviews (3): Last reviewed commit: "Rename pie slices at the display layer o..." | Re-trigger Greptile}