fix : added correct yesterday calculation in calculateStreak

[tmdeveloper007]

Closes #2463.

Summary of What Has Been Done:
Fixed a silent bug in calculateStreak (src/lib/dates/streakDateUtils.ts) where the local variable yesterday was assigned the result of getLocalDateString(userTimezone) without offsetting by one day — so the grace-period check uniqueDates[0] !== today && uniqueDates[0] !== yesterday always passed the yesterday branch trivially (the two strings were identical). The practical effect was that any user whose most recent contribution was exactly the calendar day before today in their timezone would have their streak reported as 0. Added a deterministic vitest test file covering the entire module.

Changes Made:

• src/lib/dates/streakDateUtils.ts: replaced the broken const yesterday = getLocalDateString(userTimezone) with a calculation that parses today, subtracts one calendar day in UTC, and re-formats the result as YYYY-MM-DD. The shift is anchored on local midnight to avoid DST edge cases where subtracting 24h of UTC time can land on the same date.
• New file: test/streak-date-utils.test.ts (19 tests, all passing under npm test).
• Coverage:
  • getLocalDateString: UTC, a fixed-offset timezone (Asia/Kolkata), and the invalid-timezone fallback all return a YYYY-MM-DD string.
  • utcToLocalDate: a UTC ISO timestamp is converted to the right date in Asia/Kolkata (UTC+5:30) and UTC; a Date object works; an invalid timezone falls back to the ISO date.
  • areConsecutiveDays: adjacent dates return true (forward and reverse, including month and year boundaries), the same date returns false, and dates with a one-day gap return false.
  • calculateStreak:
    • empty input → 0
    • single date equal to today → 1
    • single date equal to yesterday → 1 (regression case: the previous code returned 0)
    • single date older than yesterday → 0
    • sorted-most-recent-first array of 5 consecutive dates ending today → 5
    • a 1-day gap in the middle breaks the streak at the gap
    • repeated dates are de-duplicated
    • input order does not matter
• All other functions and call sites in the codebase are untouched.

Impact it Made:
Restores the documented one-day grace period for streaks. Eliminates a silent bug where users (especially non-UTC ones) saw their streak drop to zero at the end of the day in their own timezone. Adds unit coverage for the entire module so future changes cannot regress it silently.

[github-actions]

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