refactor(timezone): Migrate all date types to timezone-aware

[masch]

Context

Architectural decision: project uses z.date() without explicit timezone. This must evolve to timezone-aware types.

Scope

• Create Timezone type in packages/shared/src/types/
• Create timezone helpers (convertTimeBetweenTimezones, formatWithTimezone)
• Migrate reservation.ts - service dates and timestamps
• Migrate order.ts - confirmed_at, cancelled_at, completed_at
• Migrate venture.ts - createdAt, updatedAt
• Migrate user.ts - last_login_at, createdAt, updatedAt
• Migrate entrepreneur.ts
• Migrate cascade.ts
• Update backend API to accept/send timezone
• Update tests

Dependencies

• Issue feat(catalog): define time ranges for each moment #165 - create timezone base in shared

Priority

Medium

Labels: enhancement, backend, frontend