Switched all audit and creation timestamps from UTC to local time
by replacing DateTime.UtcNow with DateTime.Now across the codebase.
This affects audit fields, object creation, and default values for
date/time properties.
All test files and utilities now use the EnvelopeGenerator.Tests.Application namespace for improved organization and clarity. No functional changes were made; updates are limited to namespaces and using directives. This makes it explicit that these are application-level tests and related helpers.
