Crowdfunding: Claim challenges

This module adds a way for logged-in portal users to take ownership of a crowdfunding challenge directly from the website.

• On the challenge detail page, a Claim button appears in the navigation area, but only when the challenge is still open and has not already been claimed by someone else.
• Clicking Claim assigns the challenge to the current user's contact and moves it to the claimed state.
• After a successful claim, the page is shown again with the discussion section hidden, so the new owner sees a clean confirmation view.
• If the challenge is already claimed or is not open, the claim page returns a not-found error instead of allowing the action.

The module extends the existing crowdfunding.challenge model with the claiming logic and reuses the standard challenge detail templates from the crowdfunding module. It does not add new menus or models of its own; the feature is exposed through the existing challenge page on the website.