Fix Safari iOS swipe-back gesture causing screen flicker

[Beamanator]

Explanation of Change

Refines the edgeDragGestureMonitor patch for @react-navigation/stack to more reliably detect iOS Safari swipe-back gestures.

Root cause: When a user swipes from the left edge in Safari iOS to navigate back in browser history, the React Navigation spring animation briefly renders the previous screen before completing, causing a visible flicker.

Previous approach: Detected the gesture by checking if touchend.pageX < 0 (negative x coordinate). This was unreliable on newer Safari versions where the coordinate is not negative.

New approach: Records touchStartPositionX on touchstart, then compares against touchend.pageX. A left-to-right swipe with > 20px travel is classified as an edge gesture. When detected, the animation switches from spring to timing with duration: 0, making the transition instant and eliminating the flicker.

This also removes the resetExpectingTouchendWithDelay function and the expectingTouchend state variable, simplifying the implementation.

Fixed Issues

$ #75905
PROPOSAL:

Tests

1. Open the app on iOS Safari (or use a Safari iOS simulator)
2. Navigate to any chat or screen
3. Perform a swipe-back gesture from the left edge of the screen
4. Verify that the previous screen does NOT flicker/briefly appear — navigation should be smooth
5. Verify normal in-app back button navigation still works correctly

• Verify that no errors appear in the JS console

Offline tests

This change only affects navigation animations. Offline behavior is unaffected — navigation still works the same way offline.

QA Steps

1. Sign in to the app on iOS Safari (mobile web)
2. Navigate to any chat or report
3. Use the swipe-back gesture from the left edge of the screen
4. Verify the previous screen does not flicker or briefly appear during the transition
5. Verify that navigating back via the in-app back button still works correctly on all platforms

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I added steps for the expected offline behavior in the Offline steps section
  • I added steps for Staging and/or Production testing in the QA steps section
  • I added steps to cover failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android: Native
  • Android: mWeb Chrome
  • iOS: Native
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
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Screenshots/Videos

Android: Native

N/A - this fix is specific to iOS Safari (mWeb)

Android: mWeb Chrome

N/A - this fix is specific to iOS Safari (mWeb)

iOS: Native

N/A - this fix is specific to iOS Safari (mWeb)

iOS: mWeb Safari

MacOS: Chrome / Safari

N/A - this fix is specific to iOS Safari touch gestures