feature: add platform layer

app/assets/javascripts/services/platform.ts

@@ -0,0 +1,24 @@
+/** Platform-specific (i-e desktop/web) behavior is handled by a Platform object. */
+export interface Platform {
+  getKeychainValue(): Promise<unknown>;
+  setKeychainValue(value: any): Promise<void>;
+  clearKeychainValue(): Promise<void>;
+}
+
+const KEYCHAIN_STORAGE_KEY = 'keychain';
+
+export class WebPlatform implements Platform {
+  async getKeychainValue(): Promise<unknown> {
+    const value = localStorage.getItem(KEYCHAIN_STORAGE_KEY);
+    if (value) {
+      return JSON.parse(value);
+    }
+  }
+  async setKeychainValue(value: any): Promise<void> {
+    localStorage.setItem(KEYCHAIN_STORAGE_KEY, JSON.stringify(value));
+  }
+
+  async clearKeychainValue(): Promise<void> {
+    localStorage.removeItem(KEYCHAIN_STORAGE_KEY);
+  }
+}