feat: v0.4.1 — untrack map-asset pack (~100 MB): compose volume mount + runtime fallback to remote OFM styles

lib/mapStyles.ts

@@ -38,6 +38,33 @@ export function styleUrl(id: string): string {
   return `/styles/${id}.json`;
 }
 
+// Remote fallback base for styles when the local asset pack is absent (fresh clone
+// without `npm run assets:download` / no volume mount). The raw OFM styles already
+// point glyphs/sprites at remote URLs, so they work as-is — only vector tiles were
+// ever remote. A future self-hosted tile pack plugs into the same availability
+// mechanism below (the manifest describes the whole local asset set).
+export const REMOTE_STYLE_BASE = 'https://tiles.openfreemap.org/styles';
+
+export function styleUrlFor(id: string, localAssetsAvailable: boolean): string {
+  return localAssetsAvailable ? `/styles/${id}.json` : `${REMOTE_STYLE_BASE}/${id}`;
+}
+
+// The manifest is written LAST by scripts/download-map-assets.mjs, so its presence
+// means the whole pack (styles + fonts + sprites — and any future asset class) is in
+// place. Probed once per page life via a module-level cached promise: any non-200 or
+// network error → false (fall back to remote styles).
+const MANIFEST_URL = '/map-assets-manifest.json';
+let assetsProbePromise: Promise<boolean> | null = null;
+
+export function localAssetsAvailable(): Promise<boolean> {
+  if (!assetsProbePromise) {
+    assetsProbePromise = fetch(MANIFEST_URL, { cache: 'no-store' })
+      .then((res) => res.ok)
+      .catch(() => false);
+  }
+  return assetsProbePromise;
+}
+
 type Source = Record<string, unknown>;
 
 // Takes a raw OFM style object and returns a new one with glyphs/sprite pointed at the