Premium Analytics: Fix posting activity heatmap range

[louwie17]

Fixes WOOA7S-1693

Proposed changes

• Allow the shared calendar heatmap builder to respect explicit start/end bounds so sparse data no longer collapses the grid to the first/last active day.
• Update the Posting Activity widget to query a trailing year ending at the dashboard date range end, then render the visible slice that fits the widget.
• Use larger non-compact heatmap cells for larger tiles, while keeping compact rendering for tighter tiles.
• Add high-relevance header arrow controls backed by an activityWindowOffset attribute so smaller visible windows can page through the trailing year.
• Clamp the activity window at the latest and earliest pages so the header arrows disable at either end instead of wrapping.
• Reset the activity window offset when the measured widget size changes, so resizing/customizing returns the heatmap to the latest window.
• Add regression coverage for sparse posting activity, trailing-year bounds, size-aware paging, header arrow bounds, and resize resets.

Related product discussion/links

• WOOA7S-1693

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

No.

Testing instructions

• Run pnpm --dir projects/js-packages/charts test -- --runTestsByPath src/charts/heatmap-chart/test/build-calendar-data.test.ts.
• Run pnpm --dir projects/js-packages/charts typecheck.
• Run pnpm --dir projects/packages/premium-analytics test -- --runTestsByPath widgets/posting-activity/__tests__/posting-activity.test.tsx.
• Run pnpm --dir projects/packages/premium-analytics typecheck.
• Visual reference from a local sparse-response fixture:

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack or WordPress.com Site Helper), and enable the codex/fix-posting-activity-heat-map-pr branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack codex/fix-posting-activity-heat-map-pr

bin/jetpack-downloader test jetpack-mu-wpcom-plugin codex/fix-posting-activity-heat-map-pr

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[github-actions]

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/js-packages/charts/src/charts/heatmap-chart/private/build-calendar-data.ts	54/57 (94.74%)	-1.34%	1 ❤️‍🩹

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