fix: filter the restored view stack against RESTORABLE_VIEWS (closes #224)

The persisted view stack was restored verbatim. RESTORABLE_VIEWS stopped the
popup opening ONTO a view it will not re-render, but nothing kept such a view
out of the stack, so Back could land on a screen whose content was deliberately
never restored. No secret leaks -- those views are blank precisely because
nothing is restored into them; this is a navigation defect.

loadState() now truncates the stored stack at the first entry outside
RESTORABLE_VIEWS, dropping it and everything above it. Truncating rather than
splicing keeps the result a prefix of what was stored, so every surviving entry
keeps the Back target it had; splicing would silently re-point the entry above
the hole at a different screen. Filtering on load rather than on save is what
makes it retroactive for stacks already in storage, and leaves the live
in-session stack whole, which it should be.

The general case where Back lands on a blank screen even for restorable views,
because goBack() re-renders nothing, is separate and tracked at #268.

TODO.md

@@ -44,6 +44,14 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-12: The restored navigation stack is filtered against
+  `RESTORABLE_VIEWS` on load, truncated at the first entry the popup would not
+  render so that every surviving entry keeps the Back target it had. Back after
+  reopening can no longer land on a view the popup declined to restore, such as
+  `export-privkey` or `show-phrase`
+  ([#224](https://git.eeqj.de/sneak/AutistMask/issues/224)). Restorable views in
+  the stack are still unhidden without being re-rendered; that is tracked
+  separately in ([#268](https://git.eeqj.de/sneak/AutistMask/issues/268)).
 - 2026-08-12: One wording for a rejected password on every screen that asks for
   one — the send confirmation and the delete-wallet confirmation no longer say
   "Wrong password." (a fragment, which `RULES.md` Language & Labeling forbids)