fix: make the WebView the first responder when it loads

[dpogue]

Fixes #1132.
Maybe fixes #455.

Platforms affected

iOS

Motivation and Context

Presently, the splashscreen becomes the focused view when the app loads, and it never moves focus back to the webview. This means that the webview does not receive events (such as keypresses) until it is explicitly focused by the user (by tapping in it). We want the webview to become the focused element when the splashscreen is hidden.

Description

Upon hiding the splashscreen, set the webview as the first responder for event handling.

Testing

Tested this in iOS simulator and on macOS (with Catalyst)

Checklist

• I've run the tests to see all new and existing tests pass

[codecov-commenter]

Codecov Report

Merging #1337 (f631c8b) into master (58df099) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1337   +/-   ##
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  Coverage   78.62%   78.62%           
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  Files          15       15           
  Lines        1801     1801           
=======================================
  Hits         1416     1416           
  Misses        385      385           

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