Standardize date handling: use date-fns everywhere and remove redundant code

[TheRealSeber]

Summary

Standardize all date handling in the codebase by exclusively using date-fns. Refactor existing code to replace all custom date utilities, translations, and date-handling functions with their equivalents from date-fns. Remove any redundant or duplicate functions and translations that can be handled by date-fns.

Tasks

• Audit the codebase for all date manipulation, formatting, and parsing functions.
• Replace any custom date functions (including utilities and helpers) with date-fns equivalents.
• Remove redundant translations for date formatting/functions that are now handled by date-fns (including any i18n date formatting logic).
• Ensure consistent usage of date-fns throughout all Svelte and TypeScript files.
• Update documentation or code comments as necessary.

Acceptance Criteria

• All date logic in the codebase uses date-fns (no custom or legacy date utilities remain).
• All redundant or replaced functions and translations are deleted.
• Code is clean and conforms to project standards.

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Note: Do not use Moment.js or other date libraries. Only use date-fns for all date operations.