Premium Analytics: port the Conversion rate widget

[nerrad]

Fixes: WOOA7S-1466

Proposed changes

• Ports the Conversion rate widget into the Premium Analytics customizable dashboard.
• Registers the jpa/conversion-rate widget and renders the shared conversion-rate widget inside WidgetRoot using dashboard-level report params.
• Adds Storybook mock coverage for conversion-rate funnel data so the story renders the chart state.
• Adds standalone Default and WithComparison stories plus a dashboard WidgetDashboardWithWidget story for the widget.

Storybook screenshot

Captured from the local Storybook widget dashboard story.

Related product discussion/links

• WOOA7S-1466; parent WOOA7S-1457.
• Follows the widget port structure from Premium Analytics: port the Average items per order widget #49505.

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• CI=true /Users/dethier/Library/pnpm/pnpm install --fix-lockfile --config.confirmModulesPurge=false
• CI=true /Users/dethier/Library/pnpm/pnpm --filter @automattic/jetpack-premium-analytics typecheck
• CI=true /Users/dethier/Library/pnpm/pnpm --filter @automattic/jetpack-premium-analytics build
• CI=true /Users/dethier/Library/pnpm/pnpm --filter @automattic/jetpack-storybook storybook:build
• Verified the built Storybook index includes:
  • packages-premium-analytics-widgets-conversionrate--default
  • packages-premium-analytics-widgets-conversionrate--with-comparison
  • packages-premium-analytics-widgets-conversionrate--widget-dashboard-with-widget
• Verified the static dashboard story iframe endpoint returns 200 OK locally.

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Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

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