Feature/donation flow

[mariukle]

This PR implements the complete donation flow for GiveHope.

The flow consists of the following steps:

• If the user is not signed in, they are redirected to a "Sign in to donate"
  page with Sign In and Register buttons.
• If the user is signed in, they are redirected directly to the amount
  selection page where they can choose a preset amount (50, 100, 500, 1000 kr)
  or enter a custom amount.
• The user then enters payment information (card number, expiry date, CVC).
• Finally, the user is shown a confirmation page with the donation details.

Validation is added to prevent negative or zero donation amounts.
The navbar is hidden during the donation flow and restored afterwards.