feat: add colored Lines Changed stat tile to PR Analytics widget (#2450)

[aaniya22]

Summary

Adds a "Lines Changed" stat tile to the PR Analytics widget showing total additions/deletions across PRs in the selected time range, styled with green/red coloring to match GitHub's diff style.

Closes #2450

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Type of Change

• ✨ New feature (non-breaking change that adds functionality)

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What Changed

• Backend (src/app/api/metrics/prs/route.ts) already fetched and returned totalAdditions/totalDeletions via GitHub GraphQL — fixed a latent typeof value typo bug found while reviewing this path
• Widened PRStat.value type in src/components/PRMetrics.tsx to accept ReactNode so tiles can render styled content, not just strings/numbers
• Replaced the plain-text Lines Changed value with colored spans: green +{additions} and red -{deletions}, matching GitHub's diff coloring

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How to Test

1. Open the dashboard and view the PR Analytics widget
2. Confirm the "Lines Changed" tile shows +X in green and -Y in red
3. Switch between 7d / 30d / 90d ranges and confirm numbers update
4. Switch between PRs Authored / Reviews Given tabs to confirm no crashes

Expected result: Lines Changed tile renders with correct green/red coloring and accurate numbers across all ranges.

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Checklist

• Linked the related issue above
• Self-reviewed my own diff
• No unnecessary console.log, debug code, or commented-out blocks
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Added or updated tests where applicable
• Updated documentation / comments if behavior changed

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Accessibility (UI changes only)

• Keyboard navigation works correctly
• Color contrast meets WCAG AA standard
• ARIA labels / roles added where needed
• Tested on mobile / responsive layout

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Additional Context

Backend GraphQL fetching for additions/deletions was already implemented; this PR focuses on the frontend tile rendering and a small bugfix in the review-time calculation.

[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

[aaniya22]

The 2 failing tests (in ProfileQrModal.test.tsx and ShortcutsModal.test.tsx) are pre-existing failures on main, unrelated to this PR. I verified by running the same test files directly against main before my changes — they fail there too with identical errors. Nothing in this PR touches those files. Flagging here in case it's worth tracking as a separate issue.