Add local storage

frontend/allowance-planner-v2/src/app/services/storage.service.ts

@@ -0,0 +1,28 @@
+import { Injectable } from '@angular/core';
+
+import { Storage } from '@ionic/storage-angular';
+
+@Injectable({
+    providedIn: 'root'
+})
+export class StorageService {
+    private _storage: Storage | null = null;
+
+    constructor(private storage: Storage) {}
+
+    async init() {
+        // If using, define drivers here: await this.storage.defineDriver(/*...*/);
+        const storage = await this.storage.create();
+        this._storage = storage;
+    }
+
+    // Create and expose methods that users of this service can
+    // call, for example:
+    public set(key: string, value: any) {
+        this._storage?.set(key, value);
+    }
+
+    public async getCurrentUserId() {
+        return await this._storage?.get('user-id');
+    }
+}