fix(ios): apply background color behind status bar

[skyfallsin]

Summary

ios.backgroundColor was applied to Capacitor's WKWebView, but not to the visible top safe area behind iOS's transparent status bar. Apps could configure a native background color and still see the system fallback color above their content.

This adds a non-interactive native view between the WebView's top edge and its top safe-area anchor. It uses the configured ios.backgroundColor, falling back to systemBackground when no color is configured.

Visual verification

A real Capacitor app in the iOS Simulator was configured with:

ios: {
  backgroundColor: '#ff0000',
}

Released framework	This change
The top safe area stayed at the system fallback: rgb(246,247,250)	The top safe area used the configured color: rgb(255,0,0)

Screenshots are available in the internal verification record; they can be attached here if needed.

Testing

• Added testConfiguredBackgroundCoversStatusBarSafeArea, which verifies that Capacitor installs the configured, non-interactive native status-area view inside the WebView.
• Passed CapacitorTests/testConfiguredBackgroundCoversStatusBarSafeArea on an iOS Simulator: 1 test, 0 failures.
• Built the Capacitor iOS framework successfully.
• Verified the result in an iOS Simulator using a real Capacitor app and the patched framework.
• git diff --check passes.

Related issues

• bug: iOS WebView background color configuration option not showing due to background isOpaque setting #5335 fixed ios.backgroundColor for the WebView itself. This change covers the remaining visible top safe-area region behind the transparent iOS status bar.
• Setting backgroundColor in capacitor.config.json does not work #1901 is an earlier report that backgroundColor appeared ineffective on iOS.
• [Feature]: change native background in runtime #7389 requests changing the native background at runtime. This PR intentionally addresses only the existing static configuration; it does not add a runtime API.
• Change Statusbar background or text colour on iOS #2147 and StatusBar.setBackgroundColor(color) for iOs or Plugin #777 describe the lack of an iOS StatusBar.setBackgroundColor API. This uses the documented native WebView background configuration rather than changing that API.