feat(charts): time axis for line charts with date X columns (#1159)

[ZhuchkaTriplesix]

Closes #1159

What was wrong

The line chart used the row number as X, so dates were spaced evenly regardless of the real gaps, and the axis did not show time.

Changes

• ChartData.parseTime reads ISO dates and timestamps (with or without a time or zone). isTimeColumn decides when the X column is a time column.
• For a line chart over a time column, points are sorted by time, each keeps its time (also through aggregation), and sit at their distance in days from the earliest point. Other X columns keep row order.
• On screen, the bottom axis shows date ticks: hours under two days, dates under about 13 months, months beyond. The tooltip names the nearest point.
• The SVG export places points the same way and labels six dates along the axis.
• Tests: parsing and time-column detection; the issue's example (gaps 1 and 58 days, ratio 58); time order; non-date X unchanged; tick formats by range; SVG labels.

Not in this PR

• The column type from the result grid (columnDataTypes) is not used; detection parses the cells. The issue allowed either.
• Bar and pie charts keep their label axis; the issue asks for a line chart time axis.

Not verified locally

Tests were not run locally, per the project rule.