Enable time zones selection in accounting

🔍 Problem Statement

As an accounting admin, on-screen data is in ET while downloaded reports are in UTC, so cash receipts and earnings don't tie out. Today the only way to change the timezone is via Stripe settings — which leaves clients without a payment-gateway integration with no way to adjust it at all.

💡 User Story

As an accounting admin, I want to select my own display timezone in Pelcro independently of any payment gateway, so that on-screen and exported timestamps match my region.

🎯 Definition of Done (DoD)

• ✔ Given Settings → Accounting, when an admin selects a timezone, then all on-screen accounting/reporting timestamps and every exported CSV honor that timezone.
• ✔ Core API report-export endpoints accept a timezone parameter (or expose the configured display timezone) while keeping stored timestamps in UTC.
• ✔ This change will impact UI / API — specifically Settings (Platform UI), the Accounting/Reports pages, and the Core API report-export endpoints.
• ✔ Limitations: timezone only affects display/exports; underlying stored timestamps remain UTC; not tied to Stripe or any payment-gateway config.