feat(charts): time axis for line charts #1159

Problem

The line chart uses the row number as X (FlSpot(i, value)); X labels are strings. Dates and timestamps are spaced evenly no matter the real gaps, and the axis does not show time.

Scope

• Detect date / timestamp X columns (from columnDataTypes when available, otherwise by parsing ISO dates).
• Time X axis: points at real positions, tick labels with a format picked from the range (hours / days / months), sorted by X.
• Same in SVG export.

Acceptance

• Points 2026-01-01, 2026-01-02, 2026-03-01: the gap between the 2nd and 3rd is ~58× the first.
• Non-date X keeps today's behaviour.
• Unit tests for detection and tick formatting.