feat: add configurable timezone support

[ralsina]

Summary

Add timezone configuration to address issue #10 where timestamps were displayed in UTC requiring mental math conversion.

Features

• Command line option: --timezone with IANA names and GMT offsets support
• Environment variable: GRAFITO_TIMEZONE for configuration
• Default to local timezone for better user experience
• IANA timezone names: America/New_York, Europe/London, etc.
• GMT offsets: GMT+5, GMT-3:30 with minutes support
• Graceful fallback for invalid timezones with warning logs
• Timezone-aware timestamp formatting in log display

Usage Examples

# Command line
./bin/grafito --timezone America/New_York
./bin/grafito --timezone GMT+5

# Environment variable
export GRAFITO_TIMEZONE="Europe/London"
./bin/grafito

# systemd service
Environment="GRAFITO_TIMEZONE=America/New_York"

Test Plan

• All existing tests pass (75 specs)
• Added 7 new timezone-specific tests
• Build succeeds without errors
• Linting passes with Ameba
• Manual testing with various timezone formats
• Graceful fallback for invalid timezones
• Default behavior uses local timezone

Closes #10