feat(changelog): remember the newest change the reader has seen

First-run setup marks a new reader up to date, since changes mean nothing
before you've used the app. An absent marker therefore means an install
from before the changelog, which is what lets the launch be announced to
existing readers once.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/changelog.test.ts

@@ -63,3 +63,22 @@ describe("what a reader has not seen", () => {
     expect(compareUrl("https://r", "a1b2c3d", "e4f5a6b")).toBe("https://r/compare/a1b2c3d...e4f5a6b");
   });
 });
+
+describe("the seen marker in prefs", () => {
+  test("restore keeps a string and drops anything else", async () => {
+    const { restorePrefsValue } = await import("../src/client/state/usePrefs.ts");
+    expect(restorePrefsValue({ changelogSeen: "check-in-goals" }).changelogSeen).toBe("check-in-goals");
+    expect(restorePrefsValue({ changelogSeen: 7 }).changelogSeen).toBeUndefined();
+    expect(restorePrefsValue({}).changelogSeen).toBeUndefined();
+  });
+
+  test("first-run setup marks everything seen", async () => {
+    const { firstRunPrefs } = await import("../src/client/state/usePrefs.ts");
+    expect(firstRunPrefs(["genshin", "hsr"], ["hsr"], "soon")).toEqual({
+      onboarded: true,
+      hiddenGames: ["genshin"],
+      view: "soon",
+      changelogSeen: newestId(ENTRIES),
+    });
+  });
+});