Test/streak calculation unit tests

[palakjaiswal16]

Summary

Adds comprehensive unit tests for the streak calculation utility functions and moves the streak calculation logic into a shared reusable utility.

Closes #1069

Type of Change

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

Changes Made

• Extracted streak calculation logic into src/lib/streak.ts
• Updated the streak metrics API route to use the shared utility
• Added 10 focused unit tests covering current streak, longest streak, missed days, streak freezes, at-risk detection, timezone boundaries, leap years, and month length edge cases
• Fixed the existing clipboard mock setup in StreakTracker.test.ts so tests pass in the current Vitest environment

How to Test

Steps for the reviewer to verify this works:

1. Run npm test
2. Run npm run type-check
3. Confirm test/streak.test.ts passes and covers the requested streak calculation scenarios

Screenshots (if UI change)

Not applicable. This PR only updates utilities and tests.

Checklist

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

[vercel]

@palakjaiswal16 is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[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