WOOA7S-1509: Port Stats Most popular day widget

[kangzj]

Fixes WOOA7S-1509

Proposed changes

• Port the Jetpack Stats Insights "Most popular day" highlight into a registered Premium Analytics widget (jpa/most-popular-day).
• The widget shows the day of the week that draws the highest share of views (e.g. "Thursday") together with that share, composing a <Text> label with MetricWithComparison (value only, no comparison) from the widgets toolkit.
• Data comes from the existing useStatsInsights hook. The insight is site-wide: it has no comparison period and does not depend on the dashboard date range, so WithComparison renders identically to Default.
• Ships the required widget files (package.json, widget.json, widget.ts, render.tsx, CSS Module) plus the three-story Storybook file. stats/insights is not covered by the central registerReportMocks(), so a story-scoped apiFetch middleware supplies the raw insights payload (the shared mocks are left untouched).

Related product discussion/links

• WOOA7S-1509

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

No. It reuses the existing stats/insights proxy endpoint through the established data hook; no new tracking.

Testing instructions

• Live-dashboard verification pending — handled centrally; Storybook + audit + eslint + build pass locally.
• Storybook: run the Premium Analytics Storybook and open Packages/Premium Analytics/Widgets/MostPopularDay. Confirm Default, WithComparison, and WidgetDashboardWithWidget render a populated highlight ("Thursday — 45% of views"); WithComparison should look identical to Default.
• Build/lint: jetpack build packages/premium-analytics --deps, pnpm exec eslint projects/packages/premium-analytics/widgets/most-popular-day, and stylelint on the CSS Module all pass.

[jp-launch-control]

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. 🤷

Full summary · PHP report

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[kangzj]

Live dashboard verification — Most popular day

Rendered on the connected Premium Analytics dashboard (Insights tab, raven env, blog 254382195) with real data and zero console errors.