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.

TODO.md

@@ -45,6 +45,19 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-12: "Back" now renders the screen it lands on instead of only unhiding
+  it. A reopened popup renders the wallet list and the one screen it restores
+  onto, so every screen further down the stack was still the blank template from
+  `index.html`, and Back walked straight onto it — an empty address, no
+  balances, no QR code. The Back path now goes through the same per-view
+  dispatch and data guards as the restore (`src/popup/viewRouter.js`, shared
+  with `restoreView()`), falling back to Home when the state the target would
+  render is gone, and declining any view the popup does not render from
+  persisted state, which can only be on the stack from the current page load and
+  was rendered on the way in. Forward navigation is untouched, so nothing
+  renders twice. Covered by unit tests on the real `goBack()` and by two
+  end-to-end cases against the real popup, both demonstrated failing on the
+  unfixed build ([#268](https://git.eeqj.de/sneak/AutistMask/issues/268)).
 - 2026-08-12: A containerized Firefox end-to-end harness
   (`make test-e2e-firefox`) drives the real popup in a real Firefox with the MV2
   build installed as a temporary add-on. Zero npm dependencies — a WebDriver