feat(dashboard): add commit time-of-day heatmap widget (#2118)

[IshitaSingh0822]

Summary

Implements a 7×24 commit time heatmap that visualizes when during the day/week a user codes most — similar to GitHub's contribution graph but showing time-of-day patterns instead of daily totals.

Closes #2118

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Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

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Changes Made

• Added GET /api/metrics/commit-times route that fetches commit timestamps from the last 90 days via GitHub Search API and returns a 7×24 matrix (commitTimes[dayOfWeek][hour] = count)
• Added src/components/CommitHeatmap.tsx — renders the 7-row × 24-column grid with green color scale, hover tooltip showing day + hour + commit count, and a "peak coding time" label
• Registered commit-heatmap-time widget in src/lib/dashboard-layout.ts (type union, labels, default activity section)
• Wired widget into src/components/dashboard/CustomizableDashboard.tsx (span class + switch case)
• Added "commit-times" TTL entry to src/lib/metrics-cache.ts to fix TypeScript type error

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How to Test

1. Sign in with a GitHub account that has commits in the last 90 days
2. Navigate to /dashboard
3. Scroll to the Activity & Coding section
4. Verify the Commit Time Heatmap widget renders with a 7×24 grid
5. Hover over cells — tooltip should show Tuesday 3pm — 12 commits style text
6. Verify the "You code most on..." peak stat appears below the grid
7. Verify empty (0-commit) slots render as dark empty cells, not broken
8. Hide and re-show the widget using the dashboard layout toolbar

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Screenshots (if UI change)

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Checklist

• Linked issue in summary
• pnpm run lint passes locally
• No TypeScript errors (pnpm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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Accessibility Checklist

• All grid cells have aria-label with day, hour, and commit count
• Keyboard navigation supported (tabIndex={0}, onFocus tooltip)
• Loading skeleton has role="status" and aria-busy="true"
• Responsive UI verified

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Additional Notes

This widget is distinct from the existing ContributionHeatmap (issue #18) which is a date-based calendar. This is a time-pattern matrix showing when during the day/week you code, not how much on each date.

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[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

Quality (optional):

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

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus