Migrate Cartesian website-example VR tests to automatic theme variants

Is your feature request related to a problem? Please describe. Many test-vr/tests/www Cartesian example specs manually pass testTheme: 'light'; a few add duplicate dark-mode tests with theme words in their titles. That makes coverage uneven and leaks harness concerns into test authoring.

Describe the solution you'd like Migrate the website-backed Cartesian/example stories to the automatic three-variant system from #7736. Include the Area, Bar, Composed, Line, Scatter, axis, domain/tick, reference, sizing, coordinate-system, and z-index example groups as appropriate.

Remove explicit testTheme props from migrated specs and eliminate duplicate tests whose only difference is the selected Recharts theme. Preserve meaningful non-theme props such as defaultIndex and animation controls. The gallery/test renderer, rather than each story component, must select legacy/light/dark.

Verify that each variant retains the required website CSS while keeping Recharts theme selection distinct from the website color-mode system.

Describe alternatives you've considered Continuing to add explicit light and dark calls would perpetuate inconsistent names and allow new scenarios to omit variants.

Treating website color mode as synonymous with a Recharts theme would test a different concern and produce misleading baselines.

Additional context Blocked by #7736. This issue is limited to the Cartesian-oriented www examples so snapshot updates remain reviewable; polar and general website examples are tracked separately.

Acceptance criteria:

• In-scope specs use automatic variants and contain no testTheme props.
• Existing light baselines become the reviewed light variant where semantically equivalent; legacy and dark baselines are added independently.
• Duplicate theme-only test names are removed without losing assertions.
• Website styling and variant backgrounds match the common harness contract.