Fix UTC-based date strings causing incorrect streak/quest behavior (#241)

Problem

• Date strings generated via toISOString() were UTC-only.
• Users outside UTC experienced streak resets or quest mismatches near midnight.

Solution

• Added getDateString(timeZone, offsetDays) utility in shared/utils.
• Refactored streak and quest providers to use timezone-aware dates.
• Controllers now accept X-User-Timezone header (default: UTC).
• Added Jest unit tests for multiple timezones and midnight edge cases.

Acceptance Criteria

• ✅ Dates reflect user’s local time.
• ✅ Streak/quest continuity preserved across timezones.
• ✅ Centralized date logic prevents duplication.
• ✅ Backward compatibility maintained.

Testing

• Unit tests for UTC, Asia/Kolkata, America/New_York.
• Midnight edge case simulated with mocked Date.
  Closes Fix UTC-based date strings causing incorrect streak/quest behavior for non-UTC users #241