fix: wipe the exported private key from the DOM on any view leave (closes #221)

TODO.md

@@ -44,6 +44,12 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-12: The private key export screen now wipes the key from the page
+  whenever it is left by any route, and a decrypt still in flight when the
+  screen is left is discarded instead of written; the same `onViewLeave()`
+  cleanup was extended to every other screen holding secret material in the DOM
+  (AddWallet, ConfirmTx, DeleteWallet, ApproveTx, ApproveSign)
+  ([#221](https://git.eeqj.de/sneak/AutistMask/issues/221)).
 - 2026-08-12: An xprv wallet already in storage that was imported from a
   non-master key is detected from the depth of its stored `xpub`, explained in
   the wallet list, and blocked from signing, sending and private-key export