Dataset chart: visualise the second dimension as grouped series (multi-dim charts) #1759

Problem

Dataset-bound chart rendering (introduced in #1757 / objectstack-ai/objectstack#1890) only honours the first dimension. When a chart's dataset selects two dimensions (e.g. dimensions: ['status', 'priority']), queryDataset correctly returns the full status × priority grouping, but the renderer maps xAxisKey = dimensions[0] and series = values, so:

• the X axis repeats the first-dimension label once per second-dimension bucket (e.g. Backlog, Backlog, Done, Done, …), and
• the second dimension is not visualised at all — no per-priority colour/group/legend.

The data is all there; the second dimension is just invisible. This is a pre-existing limitation of the dashboard DatasetWidget that the chart-view renderer (ObjectChart) inherited.

Proposed fix

When dimensions.length >= 2 (and a single measure), pivot the long-format result into wide-format and derive one series per second-dimension value:

• xAxisKey = dimensions[0]
• series = distinct values of dimensions[1] (one coloured/grouped bar series each)
• cell value = values[0]

Apply consistently in both ObjectChart (plugin-charts) and DatasetWidget (plugin-dashboard) — ideally via a shared buildChartSeries(rows, dimensions, values, fields) helper so numbers/labels stay identical across surfaces.

Edge cases to define: 2 dims + N measures (ambiguous — fall back to measures-as-series on dim0), >2 dims (use first two), missing second-dim buckets (zero-fill).

Verification

showcase Task → "Hours by Status (Chart)" (dimensions: ['status','priority']) should render grouped/coloured bars by priority with a priority legend.

Refs objectstack-ai/objectstack#1890, #1757