feat: add expense duplication functionality

[philippeckel]

• Add "Copy" and "Duplicate expense" translations in DE, EN, and FR
• Implement duplicate expense feature in edit form with URL parameter
• Add duplicate button to expense form with copy icon
• Auto-prefix duplicated expense titles with "(Copy)" suffix
• Set current date for duplicated expenses
• Hide cancel button and duplicate button in duplicate mode
• Navigate to edit page of new duplicated expense

Closes #286

[philippeckelintive]

@Petersmit27 can this be merged? Any issue left?

[Petersmit27]

I haven't come around to this yet; wanted to merge some bugfixes before adding new features. Will have a look soon