fix(security): color seed-save by outcome; explain the Encrypt button; friendly key-export errors

- Seed-backup dialog: a failed "Save to file" was drawn in the success (green)
  colour; it's now red on failure, green on success (flag reset on close).
- Encrypt-wallet dialog: the disabled Encrypt button now says why once the user
  starts typing ("Use at least 8 characters." / "The passphrases don't match."),
  and a "Show passphrase" checkbox reveals both fields.
- Key-export dialog: the raw RPC exception (e.g. "...walletpassphrase first")
  is routed through util::friendlyRpcError(), so a locked wallet reads
  "Unlock your wallet first, then try again." instead of daemon jargon.

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

src/ui/windows/key_export_dialog.cpp

@@ -197,7 +197,7 @@ void KeyExportDialog::render(App* app)
                                     s_key = key;
                                     s_show_key = false;  // Don't show by default
                                 } else {
-                                    s_error = error;
+                                    s_error = util::friendlyRpcError(error);
                                 }
                                 if (!key.empty()) sodium_memzero(&key[0], key.size());  // wipe the last transient copy
                                 s_fetching = false;
@@ -223,7 +223,7 @@ void KeyExportDialog::render(App* app)
                                         s_key = key;
                                         s_show_key = true;  // Viewing keys are less sensitive
                                     } else {
-                                        s_error = error;
+                                        s_error = util::friendlyRpcError(error);
                                     }
                                     if (!key.empty()) sodium_memzero(&key[0], key.size());  // wipe the last transient copy
                                     s_fetching = false;