SearchBar: style the typed text and drop the isEditing mirror of the focus state #26

Summary

These defects were found while reviewing #15. They were already on iOS 18 before that PR, so they are not regressions: #15 only removed the iOS 15/16 branch.

1. Typed text has no design-system style. The TextField in Components/SearchBar.swift (searchField) styles its prompt with Font.dash.subhead / black1000Alpha30, but the typed text gets no font or colour. The text you type is therefore system body (17pt) in .primary, while the placeholder is subhead (15pt). In dashwallet-ios (LocalCurrencyView, SelectCoinView), the line visibly changes size on the first keystroke. The removed iOS 15/16 branch at least set foregroundColor(Color.dash.primaryText). Fix: .font(Font.dash.subhead).foregroundStyle(Color.dash.primaryText) on the field, the same way AddressFieldView does it.
2. isEditing duplicates isFocused. @State isEditing mirrors @FocusState isFocused through an onAppear sync and an onChange copy, and the cancel button writes both. On top of that, the change is animated three times: withAnimation in onChange, .animation(value: isEditing) on the container, and again in the cancel action. Driving everything from isFocused alone removes the mirror and the double animation. Check that the cancel slide-in and slide-out animation stays the same.

How to check

Type into the search field in LocalCurrencyView. The placeholder and the typed text should match in size and colour. Then tap Cancel: the animation should run once.