Refactor data loading: move from inline JSON to client-side fetch

- Remove large JSON data injection from EJS templates
- Add client-side data-loader utility with cache-busting support
- Create server endpoints for JSON data with long-term caching
- Add graceful failure handling if core data fails to load
- Copy JSON data files to dist/data for static hosting
- Update app initialization to load data asynchronously
- Set serverAvailable flag for static builds in gulp task

This reduces HTML payload size and enables better caching strategies
for both server and static deployment modes.

server/scripts/modules/utils/data-loader.mjs

@@ -0,0 +1,53 @@
+// Data loader utility for fetching JSON data with cache-busting
+
+let dataCache = {};
+
+// Load data with version-based cache busting
+const loadData = async (dataType, version = '') => {
+	if (dataCache[dataType]) {
+		return dataCache[dataType];
+	}
+
+	try {
+		const url = `/data/${dataType}.json${version ? `?_=${version}` : ''}`;
+		const response = await fetch(url);
+
+		if (!response.ok) {
+			throw new Error(`Failed to load ${dataType}: ${response.status}`);
+		}
+
+		const data = await response.json();
+		dataCache[dataType] = data;
+		return data;
+	} catch (error) {
+		console.error(`Error loading ${dataType}:`, error);
+		throw error;
+	}
+};
+
+// Load all data types
+const loadAllData = async (version = '') => {
+	const [travelCities, regionalCities, stationInfo] = await Promise.all([
+		loadData('travelcities', version),
+		loadData('regionalcities', version),
+		loadData('stations', version),
+	]);
+
+	// Set global variables for backward compatibility
+	window.TravelCities = travelCities;
+	window.RegionalCities = regionalCities;
+	window.StationInfo = stationInfo;
+
+	return { travelCities, regionalCities, stationInfo };
+};
+
+// Clear cache (useful for development)
+const clearDataCache = () => {
+	dataCache = {};
+};
+
+export {
+	loadData,
+	loadAllData,
+	clearDataCache,
+};