[horilla] Clarify date handling architecture for multi-regional usage

### Target Module General (cross-module) ### Description Clarify and support a consistent architecture for user-facing date input and display to enable usage in multi-regional organizations with different calendar systems, without changing backend storage or business logic. ### Use Case Organizations operating in multiple countries use different calendar systems. Currently, unclear date handling across models, forms, templates, and frontend JS makes it difficult to localize date input and display without unintended side effects, limiting adoption in such environments. ### Proposed Solution Document and/or introduce clear extension points for: date entry (forms/serializers), date display (templates or JS), frontend date inputs, while keeping all internal storage Gregorian. ### Benefits Enables multi-regional and international adoption Improves maintainability and extensibility Reduces risk of unintended side effects when customizing dates Keeps core logic and database unc