Rework "nowStore" to scrollingTimeFrameStore that contains both from and to dates

frontend/src/lib/store/scrollingTimeFrameStore.ts

@@ -0,0 +1,31 @@
+import { type ScrollingTimeframe } from "$lib/types";
+import { type Writable, writable } from "svelte/store";
+
+async function createStore(): Promise<Writable<ScrollingTimeframe> & { next: Function; prev: Function }> {
+	const thism = new Date();
+	const lastm = new Date();
+	lastm.setMonth(thism.getMonth() - 1);
+
+	const { subscribe, update, set } = writable({ from: thism, to: lastm });
+	return {
+		subscribe,
+		update,
+		set,
+		next: () => {
+			update((frame: ScrollingTimeframe) => {
+				frame.from.setMonth(frame.from.getMonth() + 1);
+				frame.to.setMonth(frame.from.getMonth() + 1);
+				return frame;
+			});
+		},
+		prev: () => {
+			update((frame: ScrollingTimeframe) => {
+				frame.from.setMonth(frame.from.getMonth() - 1);
+				frame.to.setMonth(frame.from.getMonth() - 1);
+				return frame;
+			});
+		},
+	};
+}
+
+export const scrollingTimeFrameStore = await createStore();