insights: allow custom X and Y axis labels on non-SQL insights

Feature request

Is your feature request related to a problem?

Could we have the option to add custom X- and Y-axis titles to trend (and other non-SQL) insights? Enabling a clearer picture of what is being presented.

Describe the solution you'd like

Two optional text inputs, "X-axis label" and "Y-axis label", in the trend insight Options panel. When set, the strings render alongside each axis on the chart, (perhaps matching the experience that recently shipped for SQL and data visualization charts)

Describe alternatives you've considered

• Per-series custom_name already supports renaming individual series, but doesn't describe the axis as a whole.
• aggregationAxisPrefix and aggregationAxisPostfix decorate each tickvalue (for example, $10, 10ms) but aren't an axis-level title.
• The insight name and description sit above the chart, not alongside the axes, so they don't help when a chart is exported or embedded standalone.

Additional context

Ticket: https://posthoghelp.zendesk.com/agent/tickets/57867 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/58891)

Axis labels added for SQL insights here https://github.com/PostHog/posthog/pull/57546

The issue names the trend insight Options panel and points to SQL axis-label PR #57546; start by reviewing that PR and locating the non-SQL trend insight configuration and chart entry points. Done means optional X-axis and Y-axis text fields render as axis titles on non-SQL charts, including exported or embedded views.