Enable time zones selection in accounting

🔍 Problem Statement

Accounting timestamps in Pelcro use a fixed timezone that cannot be changed. Clients operating in different regions see timestamps that don't match their local business hours, causing confusion in revenue reporting and reconciliation.

💡 User Story

As a Pelcro admin, I want to select a timezone for accounting reports and timestamps, so that financial data aligns with my local business timezone.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given an accounting timezone setting, when it is configured, then all accounting reports, invoices, and financial timestamps reflect the selected timezone.