Force simulate mode when OSM OAuth secrets are missing

Preview/PR builds don't have access to GitHub Secrets, so the OAuth
client IDs are empty. Previously this caused a runtime crash from
keys.dart throwing on empty values. Now we detect missing secrets
and force simulate mode, which already fully supports fake auth
and uploads.

Also fixes a latent bug where forceLogin() would crash with
LateInitializationError in simulate mode since _helper is never
initialized when OAuth setup is skipped.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/screens/settings/sections/upload_mode_section.dart

@@ -23,6 +23,8 @@ class UploadModeSection extends StatelessWidget {
               subtitle: Text(locService.t('uploadMode.subtitle')),
               trailing: DropdownButton<UploadMode>(
                 value: appState.uploadMode,
+                // This entire section is gated behind kEnableDevelopmentModes
+                // in osm_account_screen.dart, so all modes are always available here.
                 items: [
                   DropdownMenuItem(
                     value: UploadMode.production,