Add frontend config template and gitignore

gpx-vis-frontend/.gitignore

@@ -0,0 +1 @@
+config.js

gpx-vis-frontend/config.example.js

@@ -0,0 +1,6 @@
+window.APP_CONFIG = {
+  // Backend API URL (no trailing slash)
+  apiUrl: 'http://localhost:3000',
+  // App name shown in title/header
+  appName: 'GPX Visualizer',
+};