Migrate OSM cache to localForage/IndexedDB

We don't want to hit limits with localStorage, and it's non-blocking

app/services/local-forage.js

@@ -0,0 +1,55 @@
+import localforage from 'localforage';
+import Service from '@ember/service';
+
+const DB_NAME = 'marco';
+
+/**
+ * Thin async-only wrapper around `localforage` that exposes namespaced
+ * object stores under a single IndexedDB database (`marco`).
+ *
+ * Each "store" is a `localforage` instance created with a unique
+ * `storeName`, so callers can keep data isolated (e.g. OSM cache vs.
+ * contributions name cache) without managing their own connections.
+ *
+ * All methods return Promises — IndexedDB is inherently async, so callers
+ * must `await` every read/write.
+ */
+export default class LocalForageService extends Service {
+  _instances = new Map();
+
+  _instance(storeName) {
+    let instance = this._instances.get(storeName);
+    if (!instance) {
+      instance = localforage.createInstance({
+        name: DB_NAME,
+        storeName,
+      });
+      this._instances.set(storeName, instance);
+    }
+    return instance;
+  }
+
+  async get(storeName, key) {
+    return this._instance(storeName).getItem(key);
+  }
+
+  async set(storeName, key, value) {
+    return this._instance(storeName).setItem(key, value);
+  }
+
+  async remove(storeName, key) {
+    return this._instance(storeName).removeItem(key);
+  }
+
+  async keys(storeName) {
+    return this._instance(storeName).keys();
+  }
+
+  async clear(storeName) {
+    return this._instance(storeName).clear();
+  }
+
+  async iterate(storeName, fn) {
+    return this._instance(storeName).iterate(fn);
+  }
+}