fallback and 404

svelte.config.js

@@ -9,9 +9,21 @@ const config = {
 	// for more information about preprocessors
 	preprocess: vitePreprocess(),
 	kit: {
-		adapter: adapter(),
+		// fallback: the SPA shell served for routes that were not prerendered
+		// (unlisted cities, GPS coordinate routes); the universal load then
+		// resolves the location client-side. Most static hosts serve 404.html
+		// for unknown paths automatically.
+		adapter: adapter({ fallback: '404.html' }),
 		// Pregenerate city pages to improve SEO during static build
 		prerender: {
+			// dynamic per-location routes (14-day, compare, unlisted cities) are
+			// served by the SPA fallback instead of being prerendered
+			handleUnseenRoutes: 'ignore',
+			// a transient geocoding failure for one city should skip that page
+			// (the fallback still serves it), not abort the whole build
+			handleHttpError: ({ path, message }) => {
+				console.warn(`prerender skipped ${path}: ${message}`);
+			},
 			entries: (() => {
 				try {
 					const citiesPath = path.resolve('src/routes/weather/locations/city-names100.json');