Set the dock badge in one place

The sidebar already follows the count, so the inbox and the poll don't need to set it too.

tests/unit/notificationCount.test.ts

@@ -0,0 +1,11 @@
+import { expect, test } from "vitest";
+
+import { badgeCount } from "@app/lib/notificationCount.svelte";
+
+test.each([
+  [3, true, 3],
+  [0, true, undefined],
+  [3, false, undefined],
+])("%j notifications with the badge on %j show %j", (count, on, expected) => {
+  expect(badgeCount(count, on)).toBe(expected);
+});