feat(settings): migrate Export Private Key modal + fix outside-click secret leak

Phase 2 (first security modal), presentation-only + the review's one finding.

Migration: struct-form BlurFloat overlay (was legacy positional); full i18n
(reusing export_private_key / key_export_private_warning / key_export_private_key /
select_address / key_export_fetching / export / copy / close, plus one new
key_export_failed); DialogWarningHeader; centered DialogActionFooter (Export +
Close) + a Copy TactileButton; async UX (export_in_progress_ spinner, export_error_
line). The exported key stays in the char[256] buffer shown via a read-only
InputTextMultiline (deliberately NOT AddressCopyField, which would copy the secret
into a lingering std::string). exportPrivateKey is untouched.

Security fix (found by the adversarial review): the 0a "outside-click/Esc wipe" via
the Begin-returns-false early-return was UNREACHABLE — BeginOverlayDialog clears
show_export_key_ mid-frame but still returns true, so an outside-click/Esc dismiss
left the exported spending key un-wiped in export_result_. Add the in-body
`if (!show_export_key_)` teardown before EndOverlayDialog (the proven import-dialog
pattern), which is the real wipe point for every dismiss path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/util/i18n.cpp

@@ -1393,6 +1393,7 @@ void I18n::loadBuiltinEnglish()
 
         // --- Key Export Dialog ---
         strings_["key_export_fetching"] = "Fetching key from wallet...";
+        strings_["key_export_failed"] = "Couldn't export the key \xE2\x80\x94 unlock the wallet (if encrypted) and try again.";
         strings_["key_export_private_key"] = "Private Key:";
         strings_["key_export_private_warning"] = "Keep this key SECRET! Anyone with this key can spend your funds. Never share it online or with untrusted parties.";
         strings_["key_export_reveal"] = "Reveal Key";