Premium Analytics: port Stats Referrers widget

[Nikschavan]

Fixes WOOA7S-1490

Proposed changes

Ports the Jetpack Stats Referrers module into Premium Analytics as the jpa/referrers widget.

• Adds the Referrers widget: a ranked leaderboard of websites and search engines referring visitors, backed by the Jetpack Stats API via the existing useStatsReferrers data hook.
• Supports two-level drill-down (e.g. Search Engines → Google Search → google.com), with scoped row keys so same-named rows at different levels don't collide.
• Adds a shared WidgetBackLink component and useDrillDown hook to widgets-toolkit for nested leaderboard widgets; the back link renders in the widget body while the host frame keeps the title static.
• Merges comparison-period values by scoped key before slicing primary rows, so deltas survive ordering differences between periods.
• Renders childless referrers as plain rows without outbound links.
• Handles max = 0 as "all rows", plus loading, empty, and error states per the widget guidelines.
• Adds Stats referrers fixtures to the Storybook mocks (register-stats-mocks.ts).
• Adds Default, WithComparison, and WidgetDashboardWithWidget stories.
• Adds JS unit coverage for drill-down navigation, childless rows, comparison merging, max semantics, and the new useDrillDown hook.

Related product discussion/links

• WOOA7S-1490

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

No. This reads existing Jetpack Stats referrers data through the existing Premium Analytics stats data layer.

Testing instructions

Run focused checks:

pnpm --dir projects/packages/premium-analytics exec jest --config=tests/jest.config.cjs widgets/referrers/__tests__/referrers.test.tsx
pnpm --dir projects/packages/premium-analytics exec jest --config=tests/jest.config.cjs packages/widgets-toolkit/src/hooks/__tests__/use-drill-down.test.ts
pnpm --dir projects/packages/premium-analytics run typecheck

In Storybook:

1. Open Packages/Premium Analytics/Widgets/Referrers.
2. Confirm Default, WithComparison, and WidgetDashboardWithWidget render correctly.
3. Drill into a referrer group (e.g. Search Engines), then into a source, and use the back link to return each level.
4. Confirm comparison deltas appear in WithComparison.

On a Jetpack-connected test site with referrer traffic:

1. Open Premium Analytics and add the Referrers widget from the picker.
2. Confirm referrers render as ranked rows with view counts and favicons.
3. Drill into a group with nested sources and navigate back with the back link.
4. Confirm childless referrers are plain rows, not links.
5. Select a date range with no referrer data and confirm the empty state appears.

[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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