[FEAT] Add contribution heatmap calendar view

[Priyanshu-byte-coder]

What Needs to Be Done

Add a GitHub-style heatmap (green squares calendar) alongside or as an alternative to the bar chart in the contributions section.

Why This Matters

GitHub's heatmap is the most recognizable developer activity visualization. It gives an immediate sense of consistency and streaks — much more intuitive than a bar chart for long time ranges.

Files to Create/Modify

• src/components/ContributionHeatmap.tsx — new component
• src/app/dashboard/page.tsx — add below or as tab alongside ContributionGraph
• Optionally use a library like `react-activity-calendar` or build custom with CSS Grid

Acceptance Criteria

• Heatmap shows last 90-365 days of commit activity
• Color intensity based on commit count (0 = empty, 1-2 = light, 3-5 = medium, 6+ = dark)
• Tooltip on hover shows date and exact commit count
• Responsive — scrollable on mobile
• No TypeScript errors, lint passes

Tech Context

Data source: same /api/metrics/contributions?days=90 endpoint.
For the calendar grid: CSS Grid with 7 rows (days of week) × N columns (weeks).
Color palette example: slate-800 (0), indigo-900 (1-2), indigo-700 (3-5), indigo-500 (6+), indigo-400 (10+).

[Tannuu18]

HI @Priyanshu-byte-coder
I would like to work on this issue as part of GSSoC 2026.

Please assign me this issue.