fix: Back from Settings no longer lands on a delete wallet screen left by the gear (closes #480)

Leaving the delete wallet or lost-password screen drops its wallet
selection, but the settings gear had just pushed the screen onto the Back
stack, so Back from Settings showed a screen whose button could only
answer "No wallet selected for deletion." Each screen's leave handler now
also takes it off the top of the stack, as the private key export and
recovery phrase screens do since
#461. Back from Settings then
stays on Settings once, as it does for the recovery phrase screen.

Jest tests drive the gear and then Back on both screens, and the delete
screen's own Back.

Model: opus-5-5

README.md

@@ -1838,6 +1838,9 @@ view would leave a wallet one click from deletion.
       try again." on the error line, nothing deleted
     - "I have lost my password" → **DeleteWalletLostPassword**
     - "Back" → previous screen (Settings)
+    - Settings gear → **Settings**, whose "Back" never lands back on this
+      screen: leaving drops the wallet the screen was showing, so it also takes
+      the screen off the Back stack
 
 #### DeleteWalletLostPassword (`delete-wallet-lost-password`)
 
@@ -1876,6 +1879,9 @@ view would leave a wallet one click from deletion.
       selection comes back with it. The two delete screens are siblings rather
       than parent and child: nothing is pushed on the way here, so both have
       Settings as their Back target.
+    - Settings gear → **Settings**, whose "Back" never lands back on this
+      screen: leaving drops the wallet the screen was showing, so it also takes
+      the screen off the Back stack
 - **Deliberately not password-gated.** A password in front of _discarding_ a
   secret protects nobody: an attacker at the popup who wants the wallet gone can
   uninstall the extension, so the only person such a gate stops is the owner who