Clarify desired chart behavior for downloaded HTML reports

Prerequisites

• I am using the latest version of Locust
• I am suggesting a new feature, not asking a question

Description

This issue is to agree on the desired behavior before continuing implementation work around charts in downloaded HTML reports.

Current behavior, as I understand it:

• The downloaded HTML report renders interactive charts.
• The chart toolbox can export charts as PNGs from the report/charts view.
• The existing Download Report path gives users an interactive report artifact rather than a static snapshot.

Potential user need:

There may be value in an archival/shareable report mode that preserves the chart visualization as it appeared at report-generation time, without requiring a separate manual PNG export. At the same time, replacing interactive charts with a static image would be a regression for users who rely on hover, zoom, and toolbox behavior.

Open decisions before implementation:

• Is there a user need beyond the existing interactive report plus chart-toolbox PNG export?
• If so, should the behavior be an explicit option, such as interactive report vs. static/snapshot report, rather than changing the current Download Report behavior?
• Should a report include both interactive chart data and a static image, or should it avoid duplicating those payloads?
• Should any solution live in the browser download flow, server-side report generation, or documentation?

Alternatives considered:

• Keep the current behavior and rely on the existing chart toolbox PNG export.
• Add an explicit static/snapshot report option while preserving the current interactive report.
• Improve a specific report-chart rendering bug separately if one is identified.

Related context: PR #3400 explored embedding a static chart snapshot, but the review discussion showed that the desired user behavior should be agreed before choosing an implementation.