fix: render the view "Back" lands on after the popup is reopened (closes #268)

goBack() only unhid its target. A reopened popup renders the wallet list
and the one view restoreView() lands on, so every other view is still the
blank static template from index.html: pressing Back from Settings onto an
address showed an empty address line and no balances, and the same held for
address-token, receive, confirm-tx and transaction.

The per-view dispatch and its data guards move out of restoreView() into
src/popup/viewRouter.js, and goBack() now routes a popped view through the
same code by way of a renderer index.js registers with setBackRenderer().
A view whose backing state is gone falls back to Home the way the restore
does, rather than showing an empty template. The renderer declines any view
outside RESTORABLE_VIEWS, so goBack() unhides it as before: the restored
stack is filtered against that set, so such a view can only be on the stack
from the current page load, where forward navigation rendered it on the way
in. Forward navigation is untouched and nothing renders twice.

tests/backNavigation.test.js drives the real goBack() over the reproduction
and over each of address-token, receive, confirm-tx and transaction, with
and without their backing state, and pins that a live-session view is still
only unhidden. tests/e2e/run.js adds two cases against the real popup in a
real browser — a real close and reopen, then Back onto the address screen
and onto Receive — because make check cannot see a blank view. Both were
demonstrated failing against the unmodified sources: the address line read
"" where it should have read the address.

README.md

@@ -610,6 +610,15 @@ ExportPrivKey and ShowRecoveryPhrase — are deliberately absent from that list,
 so the popup can never reopen onto one of them with no password prompt in front
 of it.
 
+A reopened popup renders the wallet list and the one screen it restores onto,
+and nothing else, so every screen on the stack behind that one is still the
+blank template from `index.html`. "Back" therefore renders its target rather
+than only unhiding it, through the same dispatch and data guards as the restore
+(`src/popup/viewRouter.js`), and falls back to Home when the state the target
+would render is gone. Forward navigation renders as it goes and does not go
+through that dispatch: rendering a screen a second time would re-fetch and
+clobber whatever it has in flight.
+
 Every screen that holds secret material in the page registers a cleanup with
 `onViewLeave()` (`src/popup/views/helpers.js`), which `showView()` runs on every
 exit from that screen rather than only on its "Back" button, so nothing secret