Introduce store helper

Responsible for serializing/deserializing data to be saved in
localStorage. Add a prefix to all localStorage entries to avoid
conflicts with other webapps. Stop guarding against localStorage
not existing, browsers can just implement a dumb interface to
disable it.

store.js

@@ -0,0 +1,46 @@
+const PREFIX = "gamja_";
+
+function getItem(k) {
+	k = PREFIX + k;
+
+}
+
+function setItem(k, v) {
+	k = PREFIX + k;
+}
+
+class Item {
+	constructor(k) {
+		this.k = PREFIX + k;
+	}
+
+	load() {
+		var v = localStorage.getItem(this.k);
+		if (!v) {
+			return null;
+		}
+		return JSON.parse(v);
+	}
+
+	put(v) {
+		if (v) {
+			localStorage.setItem(this.k, JSON.stringify(v));
+		} else {
+			localStorage.removeItem(this.k);
+		}
+	}
+}
+
+export const autoconnect = new Item("autoconnect");
+
+const rawReceipts = new Item("receipts");
+
+export const receipts = {
+	load() {
+		var v = rawReceipts.load();
+		return new Map(Object.entries(v || {}));
+	},
+	put(m) {
+		rawReceipts.put(Object.fromEntries(m));
+	},
+};