CA-6181: Workaround Keyboard toolbar redraw issue

[Phoenix7351]

Closes #246

Note

Medium Risk
Affects all in-app web text input UX by removing the system keyboard accessory bar; behavior change is intentional but may surprise sites that rely on that toolbar.

Overview
Adds a workaround for an iOS SwiftUI layout bug when keyboard focus moves from a WebView text field (with the default accessory toolbar) to a native app text field without one.

WebPageModel now builds pages with a private NoKeyboardToolbarWebView subclass that overrides inputAccessoryView to return nil, instead of a stock WKWebView. That removes the WebView keyboard toolbar so the transition no longer leaves the layout stuck, at the cost of no default Done/arrow toolbar on web inputs until a future platform fix.

^{Reviewed by Cursor Bugbot for commit 4b223c5. Configure here.}