security(console): mask key-dump command RESULTS in the scrollback

The console redacted secret command ARGS but showed the OUTPUT of key-revealing
commands (dumpprivkey / z_exportkey / z_exportviewingkey / z_exportmnemonic /
dumpwallet / z_exportwallet, and the lite seed/export verbs) in full — visible
in the scrollback, Copy All, Ctrl+C and the filter. Now, when the submitted
command is key-revealing, its result is held ONLY in a member buffer (never in
the scrollback model, so Copy-All etc. can't leak it) and a masked placeholder
is shown. An explicit "Copy secret" button routes through the auto-clearing
clipboard; the buffer auto-wipes after 60s, on Clear, and on wallet lock
(via wipeDisplayedSecrets). The command string passed to the executor is
byte-identical. New pure helper ConsoleCommandRevealsSecret is unit-testable.

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

src/util/i18n.cpp

@@ -2031,6 +2031,11 @@ void I18n::loadBuiltinEnglish()
         strings_["console_clear"] = "Clear";
         strings_["console_clear_console"] = "Clear Console";
         strings_["console_cleared"] = "Console cleared";
+        strings_["console_secret_result_hidden"] = "[result hidden — contains private key material. Use \"Copy secret\" to copy it, or it clears automatically.]";
+        strings_["console_secret_result_expired"] = "[hidden secret result cleared]";
+        strings_["console_copy_secret"] = "Copy secret";
+        strings_["console_copy_secret_tip"] = "Copy the hidden key/seed result to the clipboard (auto-clears after ~45s).";
+        strings_["console_hide_secret"] = "Hide";
         strings_["console_click_commands"] = "Click commands above to insert them";
         strings_["console_click_insert"] = "Click to insert";
         strings_["console_click_insert_params"] = "Click to insert with parameters";