feat(settings): migrate Export-Transactions-CSV modal to the reference design

Phase 1 (first modal). Bring the CSV export dialog onto the key-import reference:
- Struct-form OverlayDialogSpec / BlurFloat (was the legacy positional overload).
- material::DialogActionFooter (centered TactileButton pair) replacing the
  StyledButtons + the two ImGui::Separators.
- Full i18n: TR("close") and the success/error status now via TR (were hardcoded
  English); on success the saved path shows in a widgets::AddressCopyField with a
  green check.
- Add ExportTransactionsDialog::hide() + a modal-export-transactions sweep surface
  so the dialog is captured (it wasn't before).

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

src/app_sweep.cpp

@@ -19,6 +19,7 @@
 #include "ui/sidebar.h"
 #include "ui/windows/send_tab.h"
 #include "ui/windows/wallets_dialog.h"
+#include "ui/windows/export_transactions_dialog.h"
 #include "ui/windows/daemon_download_dialog.h"
 #include "ui/windows/xmrig_download_dialog.h"
 #include "ui/pages/settings_page.h"
@@ -266,6 +267,8 @@ void App::buildSweepCatalog()
         [](App& a) { a.import_view_mode_ = true; a.show_import_key_ = true; }, [](App& a) { a.show_import_key_ = false; a.import_view_mode_ = false; });
     add("modal-export-key", ui::NavPage::Overview,
         [](App& a) { a.show_export_key_ = true; }, [](App& a) { a.show_export_key_ = false; });
+    add("modal-export-transactions", ui::NavPage::Settings,
+        [](App&) { ui::ExportTransactionsDialog::show(); }, [](App&) { ui::ExportTransactionsDialog::hide(); });
     add("modal-backup", ui::NavPage::Overview,
         [](App& a) { a.show_backup_ = true; }, [](App& a) { a.show_backup_ = false; a.backup_status_.clear(); });
     add("modal-about", ui::NavPage::Overview,