Calendar Heatmap: Add font size and color controls for cell values and month labels

What is the problem or goal you're trying to solve or accomplish?

The calendar heatmap chart does not expose any configuration options to adjust the font size or color of the numerical values displayed in cells, or the month label text. For dashboards where cells are small or the color scheme makes text hard to read, users have no way to improve legibility from the chart editor.

How are you solving it currently?

Users can work around this using the dashboard CSS editor, targeting the SVG elements with .subdomain-text and .subdomain-label classes. However, this requires knowledge of CSS and chart IDs, is fragile across Superset updates, and is not a supported configuration path.

What is your recommended solution?

Add font size and font color controls for cell values and month/week labels to the heatmap chart's Customize tab in the chart editor, consistent with the customization options available in other chart types.