feat(repo-health): add interactive Repository Health Explorer

[singhanurag0317-bit]

Summary

Closes #398

Problem

DevTrack already calculates a Repository Health Score (commit frequency,
PR merge rate, PR turnaround, open issues, recency) but the results were
buried inside a tooltip on the health grade badge in TopRepos.tsx.
Users had no way to understand why a repo scored low or what specific
signals to improve.

Solution

New component: src/components/RepoHealthExplorer.tsx

An interactive modal that surfaces every scoring dimension individually:

Feature	Details
Grade Ring	SVG radial gauge — animated fill, score + letter grade (A/B/C)
Metric Cards	Commit freq, PR merge rate, PR turnaround, open issues, recency — each with raw value, weighted score, animated colour-coded progress bar, and actionable tip
Score Distribution Bar	Stacked bar showing each signal's contribution to the 100-pt total
Recommendations Engine	Colour-coded badges (✅ ok / 🟡 warn / 🔴 critical) with specific copy (exact counts, exact hours, days since last commit)
Trend Indicator	▲ Improving / → Stable / ▼ Declining based on overall grade
Accessibility	Escape-to-close, aria-modal, aria-label, role="dialog"

Zero new dependencies or API calls — uses the same pure scoring functions from
src/lib/repo-health.ts that were already in the codebase.

Changes

• src/components/RepoHealthExplorer.tsx (new, 248 lines): Full interactive health explorer modal
• src/components/TopRepos.tsx (+8 lines): Added selectedExplorer state; health-badge click opens the explorer modal alongside the existing RepoHealthPanel

Testing

1. Visit the dashboard and scroll to Top Repositories
2. Click any health grade badge (A / B / C) on a repo card
3. The Repository Health Explorer modal opens showing all 5 signals
4. Verify metric cards display correct raw values and progress bars
5. Verify recommendations match the actual signal values
6. Press Escape or click the backdrop to dismiss

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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