Store view builder closures lazily to prevent empty alert content

[andre-moneyforward]

Problem

AlertModifier was eagerly evaluating and storing @ViewBuilder view outputs (_actions, _message) inside init(). Because the modifier is initially constructed when item is nil, the view builders evaluated to nil and discarded the building blocks. When item was subsequently updated, the alert presented without actions or messages.
Fixes: #371

Solution

• Refactored AlertModifier to store the view builder closures ((Item) -> View) as struct properties rather than storing their evaluated view results.
• Deferred execution of the closures to body(content:), ensuring views are constructed lazily using the live, non-nil Item instance at presentation time.