feat(timezone): Create timezone utilities in shared

[masch]

Context

Base for full migration to timezone-aware (issue #170). Create reusable utilities.

Scope

• Define Timezone type as union type (America/Argentina/Buenos_Aires, America/New_York, etc.)
• Helper getTimezoneOffset(timezone: Timezone): number
• Helper convertTimeBetweenTimezones(time: string, fromTz: Timezone, toTz: Timezone): string
• Helper formatTimeWithTimezone(time: string, timezone: Timezone, locale: string): string
• Helper getDeviceTimezone(): Timezone
• Types DateWithTimezone and DateTimeWithTimezone for future use cases

Dependencies

• None

Priority

Medium

Labels: enhancement, shared