security: scrub on-screen secrets on lock; redact lite console secret args; no-shell wallet rebuild

- Locking the wallet (manual / auto / lite) now calls wipeDisplayedSecrets(),
  which scrubs + closes any revealed seed phrase, migration seed, or exported
  key — previously a locked wallet could leave the phrase/key on screen and
  resident. Runs synchronously, before the async lock round-trip.
- The console secret-arg redaction list only had full-node RPC names, so lite
  verbs (import / timport / encrypt / decrypt / unlock) echoed keys and
  passphrases into the scrollback and up-arrow history; added them.
- The offline wallet-rebuild helper was invoked via a shell string
  (popen "sh -c") built from paths including the wallet filename, so a crafted
  name could inject a command; it now refuses any path with a shell
  metacharacter (rc stays -1 and the existing verify-before-swap fails safe).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.h

@@ -114,6 +114,7 @@ class App {
 public:
     App();
     void wipeSecrets();   // scrub all resident secret buffers; called from ~App() AND the forced-exit path (L-05)
+    void wipeDisplayedSecrets();   // scrub + close any on-screen secret (seed/key displays) on wallet lock
     ~App();
 
     // Non-copyable