Premium Analytics: Fix posting activity heatmap range - #50406

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: