Add timezone support for accurate contribution streak calculation

[ayushraistudio]

Contribution streaks are often calculated using UTC, which can cause
incorrect streak breaks for users in different timezones.

We should allow timezone-aware streak calculation to make stats more accurate.

• Add a timezone query parameter (e.g. Asia/Kolkata)
• Use the timezone when calculating daily contribution boundaries
• Keep UTC as the default if no timezone is provided
• Do not break existing API behavior

/api?username=USER&theme=neon&timezone=Asia/Kolkata

Note: GitHub GraphQL data remains unchanged; timezone is applied only during streak calculation.