fix: Back from Settings no longer lands on a secret screen left by the gear (closes #461)

Leaving the private key export or recovery phrase screen drops the
selection it was showing, but the settings gear had just pushed the
screen onto the Back stack, so Back from Settings landed on a password
prompt that could only fail. Each screen's leave handler now also takes
it off the top of the stack, which is what a reopened popup already does
to these screens. Back from Settings goes to the address screen for the
export screen; for the recovery phrase screen, opened from Settings, it
stays on Settings once, as after a reopen.

Jest tests drive the gear and then Back for both screens, and
leavePrivkeyScreen() in the e2e suite expects the address screen.

Model: opus-5-5

README.md

@@ -1468,6 +1468,9 @@ view would leave a wallet one click from deletion.
     - "Reveal" (wrong password) → full-sentence error on the error line, nothing
       revealed (no screen change)
     - "Back" → previous screen (AddressDetail)
+    - Settings gear → **Settings**, whose "Back" goes to AddressDetail: leaving
+      drops the address the screen was showing, so it also takes the screen off
+      the Back stack
 - **Secret handling**: nothing is decrypted, no key is derived, and nothing is
   written into the page until the password is accepted; the key is never stored
   in state, and it is wiped from the page whenever the screen is left by any
@@ -1795,6 +1798,9 @@ view would leave a wallet one click from deletion.
     - "Reveal" (wrong password) → full-sentence error, nothing revealed (no
       screen change)
     - "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
 - **Secret handling**: nothing is decrypted or written into the page until the
   password is accepted; the phrase is never stored in state, and it is wiped
   from the page whenever the screen is left by any route, including the Settings