Add dual y-axis (y2) for line, area, bar, and combo charts

[terzioglub]

Adds an optional second value axis (y2) on the right side of cartesian charts. A y-column plots against the right axis when listed in y2.field; all other series stay on the left y axis. This lets two series on different scales (e.g. revenue in $ and conversion rate in %) share one chart without the smaller-scale series being squashed flat.

• y2 is a full axis encoding — same title, format, beginAtZero, curve, dash as y.
• Ticks and tooltip values format per axis (left uses y.format, right uses y2.format).
• Supported on line, area, bar, and combo. Axis assignment (y vs y2) is independent of shape (bar vs line via lines).
• Validation: y2.type must be number, columns are disjoint from y, and y2 is rejected with stacked, horizontal bars, or color.
• Wired through the Go model, validator, JSON schema, TSX loader, and the Recharts renderer; docs and the create-dashboard skill updated; testdata/dashboards/charts.yml combo now uses y2.

Example

```yaml
chart: combo
lines: [conversion_rate]
y: { field: [revenue], title: Revenue, format: "$,.0f" }
y2: { field: [conversion_rate], title: Conversion, format: ".1%" }
```

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
frontend/src/components/widgets/ChartWidget.tsx:1179
**Reference guides lose their axis**

When a line, area, or bar chart combines `y2` with a y-oriented reference line or band, the axes are renamed `left` and `right` while the reference retains the default axis ID, causing the configured guide to disappear.

---

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

_{Reviews (1): Last reviewed commit: "Add dual y-axis (y2) for line, area, bar..." | Re-trigger Greptile}

[greptile-apps]

Reference guides lose their axis

When a line, area, or bar chart combines y2 with a y-oriented reference line or band, the axes are renamed left and right while the reference retains the default axis ID, causing the configured guide to disappear.

Prompt To Fix With AI

This is a comment left during a code review.
Path: frontend/src/components/widgets/ChartWidget.tsx
Line: 1179

Comment:
**Reference guides lose their axis**

When a line, area, or bar chart combines `y2` with a y-oriented reference line or band, the axes are renamed `left` and `right` while the reference retains the default axis ID, causing the configured guide to disappear.

---

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

[terzioglub]

Fixed in 6e3f614 — renderRefs now takes a yAxisId and the line/area/bar cases pass leftAxisId, so refLines/refBands bind to the left axis when y2 is present (undefined and unchanged otherwise). Note combo is excluded since the validator already restricts refLines/refBands to line/area/bar/forest. Verified: a y-axis Target ref line now renders on a dual-axis line chart.

[greptile-apps]

_{Reviews (2): Last reviewed commit: "gofmt: align widget struct tags" | Re-trigger Greptile}

[greptile-apps]

_{Reviews (3): Last reviewed commit: "Bind reference lines/bands to the left a..." | Re-trigger Greptile}