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/services/auth_service.dart

@@ -36,6 +36,7 @@ class AuthService {
 
   void setUploadMode(UploadMode mode) {
     _mode = mode;
+    if (mode == UploadMode.simulate || !kHasOsmSecrets) return;
     final isSandbox = (mode == UploadMode.sandbox);
     final authBase = isSandbox
       ? 'https://master.apis.dev.openstreetmap.org'
@@ -150,7 +151,9 @@ class AuthService {
 
   // Force a fresh login by clearing stored tokens
   Future<String?> forceLogin() async {
-    await _helper.removeAllTokens();
+    if (_mode != UploadMode.simulate) {
+      await _helper.removeAllTokens();
+    }
     _displayName = null;
     return await login();
   }