fix(e2e): assert the sheets are gone rather than what sits behind them

Both post-dismiss waits checked an element behind the sheet being
dismissed — home-screen after the welcome sheet, home-account-switcher
after the manage-accounts sheet. Maestro does not occlusion-test, which
is the same property that caused the original bug (the account-switcher
tap reported COMPLETED while the welcome sheet covered it), so those
elements resolve as visible the entire time the sheet is up and the
waits passed instantly without confirming anything.

Wait on each sheet's own control disappearing instead. This closes the
window where the next tap fires while the sheet is still animating out
and gets swallowed, costing a 15s timeout and a retry attempt. Matches
the notVisible idiom already used for the paste dialog and the
forgot-password modal.

Hygiene rather than a fix for anything observed: OpenManageAccountsSheet
completed successfully in all 7 iOS retries across the 10-round soak.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: CassioMG

e2e/flows/shared/OpenManageAccountsSheet.yaml

@@ -34,9 +34,13 @@ appId: org.stellar.freighterdev
           commands:
             - tapOn:
                 id: welcome-banner-dismiss-button
+            # Wait on the sheet's own control disappearing, not on something
+            # behind it: Maestro does not occlusion-test, so the home screen
+            # resolves as visible the whole time the sheet is up and asserting
+            # on it would pass instantly without confirming anything.
             - extendedWaitUntil:
-                visible:
-                  id: home-screen
+                notVisible:
+                  id: welcome-banner-dismiss-button
                 timeout: 15000
 
       # If a previous attempt got as far as opening the manage-accounts sheet,
@@ -49,8 +53,8 @@ appId: org.stellar.freighterdev
             - tapOn:
                 id: manage-accounts-close-button
             - extendedWaitUntil:
-                visible:
-                  id: home-account-switcher
+                notVisible:
+                  id: manage-accounts-close-button
                 timeout: 15000
 
       # An "Account not loaded" toast can also overlay the top of the header.