test: add unit tests for streak calculation functions

[yachikadev]

What does this PR do?

Adds unit tests for the pure calculation functions in src/lib/streak.ts as outlined in issue #2795.

Functions Covered

calculateStreakFromDates

Empty input → returns all zeros
Single active date → current and longest both 1
Consecutive days → streak counted correctly
Gap in dates → streak breaks, longest preserved
Freeze dates → contribute to longest streak and totalActiveDays
Last active day older than yesterday → current = 0

calculateCurrentStreak

Accepts Set input
Accepts string[] of ISO timestamps and deduplicates via slice(0,10)
Empty input → returns 0

calculateStreak

Accepts Date[] and returns currentStreak and longestStreak

Timezone Edge Case

Asia/Kolkata timezone tested to verify streak stays alive when last date matches today in non-UTC timezone

Test Setup

Date.now frozen to 2026-06-24T12:00:00Z via vi.useFakeTimers for deterministic results
@/lib/date-utils mocked to isolate streak logic

Result

All tests passing ✅
Closes #2795

[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

[yachikadev]

The Playwright smoke test failure is not related to this PR.
It is caused by a pnpm-lock.yaml lockfile mismatch with the current overrides config in package.json — a pre-existing repo issue. All checks relevant to this PR pass successfully.