feat: Add Live Sprint Burn-Down Engine with Predictive Velocity Forecasting Layout

[yachikadev]

Closes #2624

What's Changed

Added a fully functional Sprint Analytics page with a live burn-down chart
and predictive velocity forecasting to the dashboard.

Files Added

• app/dashboard/sprint-analytics/page.tsx — Sprint Analytics route page with stats grid, progress bar, burn-down chart, and issue table
• src/components/SprintBurnDown.tsx — Interactive SVG/Canvas burn-down chart using Recharts with ideal line, actual progress, forecast line, and scope creep indicators
• src/components/useBurnDownData.ts — Custom React hook that computes burn-down data points, rolling velocity, and predicted completion day

Features Implemented

• ✅ Dedicated Sprint Analytics page under /dashboard/sprint-analytics
• ✅ Interactive burn-down chart with ideal burn line and actual progress line
• ✅ Dashed forecast line using rolling velocity calculation
• ✅ Scope creep indicators — visual spike when issues added mid-sprint
• ✅ Predicted completion day with delay warning if past sprint deadline
• ✅ Stats grid: completed points, remaining points, velocity, forecast
• ✅ Sprint progress bar
• ✅ Issue table with open/closed status and scope creep badges
• ✅ Mobile responsive layout
• ✅ Dashboard quick-action link added

Notes

• Currently uses mock sprint data. Real Supabase/API integration
  can be added once data schema is confirmed by maintainers.
• Page is protected by existing app/dashboard/layout.tsx auth wrapper —
  no additional auth logic needed.

[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

[Priyanshu-byte-coder]

Thanks for the contribution! We are currently in a code cleanup sprint and not accepting new features. Please see the discussion for details: #2651

This PR will be revisited once the sprint ends. In the meantime, contributions to testing, refactoring, bug fixes, or documentation are welcome.