feat(heatmap): add shared 365-day contribution calendar

[Honey-pg]

Extract ContributionHeatmapCalendar for dashboard and public profiles, paginate public contribution fetch for a full year, and align cell colors to absolute commit thresholds.

Summary

Adds a shared GitHub-style 365-day contribution heatmap calendar used on both the authenticated dashboard and public profiles. The dashboard widget is refactored to use the shared component, public profiles now show the full-year grid (with paginated data fetch), and cell colors use absolute commit thresholds with theme support.

Closes #2809

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Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing behavior)
• 📝 Documentation update
• ♻️ Refactor / code cleanup (no functional change)
• ⚡ Performance improvement
• 🔒 Security fix
• 🧪 Tests only

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What Changed

• src/lib/contribution-heatmap.ts — Shared helpers: buildHeatmap, buildMonthMarkers, date/tooltip formatters, and legend constants.
• src/components/ContributionHeatmapCalendar.tsx — Reusable 7×53 grid with month labels, hover/focus tooltips, horizontal scroll, legend, and getHeatmapCellStyle() absolute color levels.
• src/components/ContributionHeatmap.tsx — Refactored into a thin wrapper (fetch, range controls, theme toggle, drill-down) that renders the shared calendar.
• src/lib/public-profile-data.ts — fetchPublicContributions now defaults to 365 days and paginates GitHub Search (up to 1000 commits).
• src/components/public/PublicContributionHeatmap.tsx — New public profile heatmap using useHeatmapTheme() and server-provided data.
• src/app/u/[username]/page.tsx — Replaced inline PublicContributionGraph with PublicContributionHeatmap.
• test/contribution-heatmap-calendar.test.ts — Unit tests for heatmap grid helpers.
• test/public-profile-data.test.ts — Tests for 365-day default and pagination.
• README.md — Moved contribution heatmap calendar ([FEAT] Add contribution heatmap calendar view #18) from Planned to Completed.

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How to Test

1. Check out feat/contribution-heatmap-calendar and run npm install if needed.
2. Start the app with npm run dev, sign in, and open the dashboard Contribution Heatmap widget.
3. Confirm the 365-day grid renders, tooltips show date + contribution count on hover, and Default / Colour-blind theme toggle updates colors instantly.
4. Visit a public profile at /u/{username} and confirm the full-year heatmap renders with horizontal scroll on a narrow viewport.

Expected result: Dashboard and public profile both show the same GitHub-style calendar grid with theme-aware absolute color levels and accurate year-long data.

Automated checks:

npm run lint
npm test -- test/contribution-heatmap-calendar.test.ts test/useHeatmapTheme.test.ts test/public-profile-data.test.ts

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

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• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
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Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[Priyanshu-byte-coder]

New UI/feature PRs are on hold during the code quality sprint — see discussion #2651. PRs for tests, refactoring, bug fixes, docs, and performance are welcome.

[Priyanshu-byte-coder]

New feature/UI PRs are on hold during the code-quality sprint — see discussion #2651. We'll revisit after the sprint; tests, refactors, and bug-fix PRs are the priority right now.